It took 17 years for 97 percent of American mail users to scrawl or type zip codes on letters and packages.

Now the people who created the 5-digit code will introduce, next Feb. 1, the 9-number zip code, U.S. Postal Service officials said.

So instead of the 1.2 million zip codes now in use, there will be 19.8 million -- increasing the zip code directory from 1,900 pages to 30,000. The postal move has managed to resurrect a few critics who at the inauguration of the first zip code in 1963 predicted the dawning of a numeral-oriented society.

Regardless, it is clear that the days of the simpler zip codes are numbered.

The postal service defends its expansive policy by saying that the longer zip codes aren't mandatory (but neither were the shorter ones), they will speed mail services and reduce mail costs by eliminating workers.

The American Postal Workers Union is now studying the latter result to determine how many of its workers will be lost. John Morgan, president of the clerk craft division of the union, remembers many years ago when the Postal Service installed letter-sorting machines which were supposed to save money by using less workers. In the end, the union got more jobs, Morgan said.

The longer zip codes are intended primarily for use by businesses, the Postal Service's largest customers. The new zip codes are supposed to allow mail to be sorted to smaller geographic areas such as a city block or building, the Postal Service spokesman said.

The four add-on digits will be assigned to a group of residences on each side of a city block, apartment buildings, small office buildings, floors or groups of floors in large office buildings, firms that have large mail volumes and all rural routes.

Between now and Feb. 1, the Postal Service will engage in tests, training employes and more tests and the assignment of zip codes will begin on Feb. 1. The Postal Service said it expects 50 percent of its customers to use the expanded codes by 1982, 75 percent by 1984 and 90 percent by 1986.