IT IS A widespread and firm belief among guests that their departure is always a matter of distress to their hosts, and that in order to indicate that they have been pleasantly entertained, they must demonstrate an extreme unwillingness to allow the entertainment to conclude.

That is not necessarily true. If a guest rises suddenly in the middle of the fish course and vanishes without a word, the host may well worry that something is amiss... But when a guest settles in comfortably, two hours after dinner, for a conversation in which the phrase "did I ever tell you about . . ." is conspicuous, he knows that something is.

Miss Manners assures you that most hosts are quite resigned to the fact that their guests are theirs only for a defined amount of time, and accept philosophically the idea that they have homes of their own to go to.

One needn't, therefore, worry about the rudeness of admitting to this, or apologize for department in ways that suggest that if overwhelming reasons for going did not exist, one would gladly remain until the end of time.

I hate to leave, but we have a new babysitter who's only 10, and her mother made us promise to have her home on time, and she lives clear across the city, and we'll probably have to stop for gas, and Mirabelle has a working breakfast at her firm tomorrow, and I've got a big meeting -- actually, not till afternoon, but I've got to prepare for it, and Benjamin doesn't really sleep well until he knows we're home," is not the way to take leave of one's hosts.

It is the very people who babble like this who fail to understand that the reply of "Must you?" is a conventional, un-literal, rhetorical question, and who then plop down again saying, "Oh, all right, we'll just have one more quick drink" to accompany the sound of the host's heart sinking.

The way to go is to go. "What a charming evening; I'm afraid we must go now," is the proper announcement. One can only reply "Must you?"to those who understand its use; sometimes it is safer to say, "If you must -- well, it was delightful to have you here."

The only protest from a host to be taken seriously is a whispered, "Let them all go -- I need to talk to you," and then only at one's own risk.

Departure times are:

Half an hour after the last opening offer of service (coffee, after-dinner drinks), not counting offers of refills. Offers to scramble eggs for guests who were originally invited to (and fed) dinner do not count; neither does, "Shall I make a fresh pot of coffee?"

Two minutes after the first couple has sent each other exasperated looks, if you are the guest of honor, or, if there is no guest of honor, if you are the recipient of one of those looks.

One minute after the host stands up and says, "We're so glad you could come."

Not even half a minute after the guest has been given his coat and is standing in the hallway with his hosts -- no matter how many clever, complimentary and explanatory remarks have just occurred to him. MISS MANNERS RESPONDS

Q. I'm finding that the new poor are raising etiquette problems never addressed by the old experts.

Recently, I found myself in just such a quandary after enjoying a single-serving bottle of orange juice on a city street. A grizzled street person was rifling the sidewalk garbage can as I approached to discard my bottle.

Feeling slightly awkward, I murmured, "Excuse me," and threw the bottle in.

Should I have offered the bottle to this gentleman first -- after all, it still contained a little bit of orange juice. Or should I merely have waited until he had finished eating out of the trash? Please advise.

A. Thank you for brightening Miss Manners' day with a question that requests neither tricks of ingratiating oneself with those whose favor one courts, nor put-downs for those who get in one's way. That is true charity.

Dining from a trash can is not considered a practice of choice. It is therefore polite to refuse to recognize the implications of the act. Miss Manners prefers to assume that the gentleman inadvertently dropped his lunch into the can when he discarded his newspaper, and has decided to rescue it for the sake of the dear one who lovingly packed it for him.

She would not, therefore, offer a drop of orange juice as charity; nor would she spill it by throwing it on his spread.

She would inquire, "Would you be so kind as to throw that away for me?", hand it to him, and walk off without observing what he decided to do with it.

Q. This is my third marriage, and I am determined to try the new monogamy. I have been in an open marriage -- declared open with everything spelled out and the policy of being honest about what you were doing -- and I have been in the more traditional type of marriage, where everybody fools around and tries not to get caught.

What I want to know is how I explain my new situation to people who don't understand, and expect something different, without hurting anybody's feelings. I don't want them (I'm talking about men who might happen to be attracted to me) to think it's anything in them personally, or to feel rejected.

I could, of course, tell them about my decision, but I think it would sound too judgmental. Some of them might be married, too, and I wouldn't want them to think that I disapprove of their behavior in asking me.

If I say I'm afraid of getting herpes, which is a factor, too, as it would be hard to explain to my bridegroom in addition to everything else, that might lead to a discussion of whether they have it, and if they don't and can prove it, then what?

There must be a tactful expression to use in these situations. I just never had occasion to learn it before. What kind of excuses did proper ladies used to give?

A. A great many, but they gave them before they were about to practice sin, not virtue, which is not thought to require excuses or apologies. A simple refusal will do. Miss Manners hopes you do not find this information unbearably judgmental.