It may be just a matter of time before your ZIP code stretches as long as your Social Security number.

Despite some initial rumblings from Congress and the Office of Management and Budget, the Postal Service is ready to go forward with the voluntary nine-digit ZIP. Earlier this month, it began the task of notifying 15 million businesses and post office box holders of their new expanded numbers.

It will be a while, though, before businesses will be able to enjoy a discount for using the new ZIP code. A provision of the budget reconciliation act prohibits the Postal Service from offering the discount until after October, 1983, and the Postal Service yesterday pulled back its discount rate request before the Postal Rate Commission until closer to that date.

But, in the meantime, the service is allowed to "disseminate information" about the code, including lending computer tapes to volume mailers to help them convert their mailing lists. Homeowners will not be notified of their new ZIPs for several months.

It's been a long haul since 1975, when a five-member task force from the Postal Service's regulations and engineering divisions tried to figure out a way to speed up the mails. "In effect, we locked them in a room up here on the 10th floor and told them to come up with something," said James V. Jellison, senior assistant postmaster general.

Their answer was the nine-digit ZIP, or "ZIP+4" as the Postal Service calls it, to emphasize that there are really only four new numbers to learn, not nine.

The strategy for speeding the mails will combine the ZIP code with $887 million worth of new equipment: optical character readers at central offices will read ZIP codes that are typed or printed and translate them to a bar code like the ones found on grocery store products, which the machine will print on the lower right corner of the envelope. When the letter reaches its destination office, another machine will read the code and sort the mail accordingly.

Currently, mail is handled by workers who read the five-digit code and type it on a keyboard. The machines direct the mail to its destination, where it is again sorted by hand for the carrier.

The current generation of machines can handle 1,850 pieces of mail per worker-hour; the new equipment will be able to handle between 4,000 and 10,000 pieces, postal officials say.

Had the machines and the new ZIP been in effect last year, Jellsion said, the 106.5 billion pieces of mail processed by the Postal Service could have been handled by 15,000 fewer people.

He contends that doesn't mean many jobs will be cut when the new system is in place, rather that the Postal Service will "just add fewer people as our volume increases."

Critics, however, dispute whether projections of increased volume will hold up with increased competition from alternative systems like United Parcel Service and electronic mail.

But most of those who think the extra four digits will add up to more trouble than they're worth agree that the automation is badly needed.

To counter complaints about the nine-digit ZIP, postal officials emphasize that it is aimed at business mailers, who generate 84 percent of the mail. Personal mail, even letters that don't carry the longer ZIP, will not be delayed, they contend, because if major mailers use the code, the total volume will move more quickly.

They were able to convince OMB, which had questioned whether the cost of converting to the new ZIP code would outweigh its benefits. Postal officials convinced the regulatory reformers that service would not deteriorate for persons who didn't use the ZIP.

But some businessmen have complained about the cost of converting their mailing lists to include the new ZIP. The Postal Service estimates this will involve a one-time cost of from one-half to two cents per address. The proposed discount rate for bulk mailers would be a half-cent per letter for mailings of more than 500 pieces.

Other businessmen challenge whether the ZIP is truly "voluntary" when they will have to use it to stay competitive.

Jellison said the Postal Service sees "tremendous potential" for the longer ZIP. In the future, he said, the machines may be able to use the code to sort the mail right down to the order in which the carrier should deliver it.