Assuming that you are an average male, you shave and you're about to contemplate your visage in the mirror this morning, you should know that:

* Someone has counted, and you'll be waging an attack on something like 15,500 whiskers.

* The little critters grow 15 thousandths of an inch a day.

* Hair covers about a third of a square foot of your face.

* You'll spend about 3,350 hours scraping off 27 1/2 feet of whiskers during your (shaving) lifetime.

That translates to about five months. But before you lament all that time shaved off your life, think about the fastidious types like Prince Charles who shave twice a day. And how about all the reflecting that goes on in front of your mirror?

Most men, you'll be heartened to hear, shave themselves. Even the high and the mighty.

"I have never shaved a president in my life," says presidential barber Milton Pitts, 64.

He did, however, offer his services to President Reagan at the time of the assassination attempt. Reagan shaved himself with an electric razor.

"President Nixon always shaved himself in the morning with a regular safety razor. In the evening, if he was going out, he would use an electric razor. The same was true with President Ford."

Pitts, who has had a barbershop in the Sheraton-Carlton Hotel for the last 17 years, also works from a small shop in the White House. "All the Cabinet comes to me now," he says, "but none of them ever gets shaved.

"It's not a question of the economy or anything like that. It's a time-saving and convenience thing."

Nonpresidential shaves?

"I shaved Henry Kissinger a number of times when he would be coming off a trip abroad. A number of years ago I used to shave four or five people in the morning before I'd ever do a haircut. No more."

Customers asking for a shave now number around one or two a week. Usually, says Pitts, "It's someone whose baggage has been lost on an airplane or something and he's from California."

Says a barber at the Watergate Barber Shop: "The people asking for a shave either have been drinking, they don't feel well or they have trouble shaving themselves. We don't get more than one or two a week."

"People used to get shaves," says Harry M. Wall, 56, proprietor of Wall's Barber Shop, "but no more today. If we get one shave a week, we're lucky."

Wall, who has been barbering for the past 34 years (including 16 years at his present 15th Street NW location), says barbershop shaves have been on the decline for the past 15 or 20 years.

Just as well, it would appear.

"I've seen many barbers, myself included," Wall says, "who ran out of the shop when tough shaving customers would come in. I mean, ran out of the shop!

"As far as I know, barbers never did like to shave people. It takes time. They would rather cut a head of hair, because where a head of hair may take 15 or 20 minutes, a shave may take half an hour.

"And people expect to get shaved cheap. It's not that way today. You charge according to the time consumed." (Shaves in Washington-area barbershops range from $3 to $10. Remember shave and a haircut, two bits?)

"If I had a choice of whether or not to shave? Hell, no; I'll put it to you that way.

"I had a customer who used to come in years ago. His name was Harry, like mine. He'd say, 'Harry, do you think I need a shave?' This man was hard to shave, and I'd say to him, 'No, Harry, you don't need a shave.' And he'd have a beard!"

So today, it's every man for himself. And whatever your style, there's a wide array of tools out there: Besides the old-fashioned straight razor, our arsenal includes double-edged, single- and double-bladed safety razors, rotary and oscillating electric razors, even wind-up razors.

If you're part of the 90 percent of all men in the United States who shave regularly and you use a blade, you have a lot of company, 56.2 million, with 19.5 million using electric shavers.

The "dry" (electric shaver) and "wet" industries both claim--although they're careful not to belittle the other's products--to offer the ultimate in shaving. (Actually, the two methods give almost equally close shaves, with the difference only a couple thousandths of an inch in the blade's favor.)

Both sides agree generally on pre- and post-shave technique and recommend sticking with one method or the other.

"Combination" razors--electric shavers used with a wet lather--marketed outside the United States are a fad and "represent the worst of both worlds," claims Peter Williams, director of research and development for Warner-Lambert Co.'s (Schick) shave products.

The most important aspect of wet shaving, he says, is "to spend adequate time softening your beard."

Gillette Co. spokesman Greg Niblett agrees, noting that pre-washing the face helps remove dried oil and perspiration as well as flakes of dead skin. Washing your face with warm water for at least two minutes before shaving, he adds, allows facial hair to swell (up to 34 percent), making it softer and easier to remove.

And if you're the type who bounds out of bed and shaves immediately, Williams could waggle a finger at you."The ideal thing is to get up and perform some other function for as long as you can, up to 30 minutes. But 10 minutes is better than no wait at all."

That puffiness you've tried not to notice in the mirror is the result of a buildup of fluids during sleep. You should give them, says Williams, a chance to drain before shaving.

Shaving with a safety razor scrapes off portions of the top layer of facial skin along with whiskers: another reason for softening the beard and for using a good shave cream.

" 'Good,' " says Williams, "is any shave cream that takes your fancy."

The cream helps keep moisture on the face; its lubricants help the blade glide smoothly. Other ingredients help soothe abrasions as you go along.

Which brings us to an advantage of electrics cited by J. Richard Gonzalez, a vice president of North American Philips Corp. (manufacturer of Norelco electric shavers). Electrics, he says, "only cut the hairs that come up through the shaver head guards, what we call the combs. The blades never touch the skin, so you don't get the cuts and nicks."

"If a person has something like a mole or facial bumps," notes Milton Pitts, "you never have a chance to cut yourself as you will with a blade. They have such good electric razors now."

Whereas a wet, soft beard is important for wet shaving, Gonzalez says a dry, soft beard is the key to a good electric shave. "Drying makes the beard stand up."

If you've been shaving with a safety razor and get an electric for Father's Day, "That first shave," warns Gonzalez, "is not going to be as good as a blade shave. It takes a week or two. If you normally get a 7 o'clock shadow, you'll get a 4 or 5 o'clock shadow for the first few days."

That, he says, is because it takes a few days for the skin to slough off the horny, callous layer that comes from daily scraping of the face with a blade. Once that has been accomplished, "Your skin," declares Gonzalez, "will be much smoother and you'll be getting a dynamite shave."

The "wets" agree that the ultimate shave may not be the closest shave, but you're in control: Go with the grain for the most comfort; against the grain for the closest shave.

Whatever your method, you and all those other average American males will be whisking daily for 55 years . . . unless you hark to the hirsute example of one North Dakotan, Hans Langseth, whose beard was 17 1/2 feet long when he died in 1927.

The beard was resurrected from the family attic and presented to the Smithsonian Institution in 1967. It is now on view in the Museum of Natural History.