You won't be through with taxes for the year even if you beat the midnight Monday deadline for filing your federal return. The average American will have to work through May 5 to satisfy the tax collectors. If that prediction by the Tax Foundation proves accurate, it will be the latest ``Tax Freedom Day'' on record and two days later than 1989. The reason is simple, the nonpartisan research organization said Sunday in announcing the mythical date: ``Tax increases will outpace the growth in individuals' income during 1990.'' Tax Freedom Day is the foundation's estimate of how long it would take an average person to pay his or her state, federal and local taxes if all income went for taxes until they were all paid for the year 1990. The calculations assume that all taxes are paid by individuals, including those collected from corporations. Until this year, the latest date was May 4, 1981, before a big tax reduction took effect. The foundation said subsequent watering down of several deductions, increases in Social Security taxes and state and local taxes, and a gradual economic slowing will have wiped out that reduction. For the millions of couples and individuals still struggling with 1989 returns, the Internal Revenue Service announced that its toll-free telephone service would remain open late Monday night to answer technical tax questions. The Postal Service said most post offices in cities with at least 30,000 population planned to station clerks at curbside to receive returns. Neither the IRS nor the Postal Service estimated how many returns were likely to be filed Monday night. However, the IRS said it expects to receive about 23 million this week _ or one of every five that will be filed this year. About 6 million couples and individuals unable to file their returns on time were expected to receive a four-month extension by filing Form 4868 instead. The extension is automatic _ but only if Form 4868 is accompanied by a check for estimated taxes owed. Another 650,000 or so Americans abroad, including military personnel, qualified automatically for a two-month extension just by having their main business, home or duty station outside the United States and Puerto Rico. A number of people have tax years that ended on days other than Dec. 31, 1989, and thus have other filing deadlines. Still others will simply miss the filing deadline, do nothing about it and subject themselves to separate penalties for filing late and paying late. Taxpayers who file their returns with the IRS Service Center in Andover, Mass. _ residents of New England and most of upstate New York _ have until midnight Tuesday to file. That is because Monday is Patriots Day, a legal holiday, in Massachusetts. When all the returns are counted later this year, they are expected to total 111 million, an increase of about 1 million from 1989. About four of every five returns are qualifying for refunds, slightly above last year's figure. Refunds are averaging more than $850. If IRS estimates of how long it takes to prepare a return are correct, procrastinators should not wait too late Monday before beginning the task. The agency guesses the average person requires 2 hours and 32 minutes to learn about the law and Form 1040, another 3 hours 10 minutes to fill it out, and 35 minutes to copy, assemble and mail it. That adds up to 6 hours and 17 minutes _ not counting the 3 hours and 7 minutes of recordkeeping required. And that total does not include the time required to do a Schedule A for itemized deductions (1 hour and 47 minutes plus recordkeeping), a Schedule B for interest and dividends (44 minutes) or any other supplemental form. While the conservative Tax Foundation was highlighting the overall growth of taxes, the Democratic Study Group was marking the filing deadline by decrying a shift of tax burden away from the rich. The organization, which includes most liberal Democrats in the House, released an analysis indicating that since 1980, the total federal tax burden for the three-fifths of the population with low to middle incomes rose, the next highest 20 percent realized a net cut of only about $45 and ``the 1 percent of households with the very highest incomes ... (had taxes cut) by an average of more than $12,000.''