Once the Monday deadline for filing 1988 federal income-tax returns passes, the average worker still will have to toil an additional 17 days before having earned what government tax collectors will demand for 1989.

"Tax Freedom Day" arrives this year on May 4. It has not been later since the Tax Foundation began calculating this measure of the burden imposed on workers by federal, state and local governments. It was Feb. 13 in 1930.By the foundation's calculations, the average worker will work 124 days this year just to pay taxes. So, if the government demanded its money up front, it would take all of the average worker's income through May 4.

That is the same as the past two years - in fact, it is the same as in 1981, when Congress and President Reagan agreed on the biggest tax cut in the nation's history.

"Americans are not better off than they were in 1981, despite economic growth, cuts in income tax rates and repeated claims that government spending has been cut to the bone," the foundation said Thursday.

The reasons: 13 federal tax increases enacted since 1981; the giant 1986 tax overhaul, which, to millions of taxpayers, took away more than it gave; Social Security taxes that have been growing more rapidly than the economy; and steady state and local tax increases caused in part by a reduction in federal aid.

If you prefer worrying about your taxes on the installment plan, the foundation offers this option: The average worker will put in 1 hour and 46 minutes each 8-hour day this year to pay federal taxes and another 56 minutes for state and local taxes. The federal take is down by a minute from 1988; the state-local burden is unchanged.