What is important to realize about President Reagan, now that he has finally translated his repeated warnings to terrorists into military action against Libya, is that he has become acutely conscious that time is running out on his presidency.

Reagan is a fatalist who rarely expresses intimations of mortality or reflects on the limitations of presidential power. He scorns the notion that he is a "lame duck." Yet in a nationally televised speech March 16, he said, "I have only three years left to serve my country, three years to carry out the responsibilities you have entrusted to me, three years to work for peace."

The speech was a plea for military aid to Nicaraguan rebels, a proposal now diverted into in the parliamentary limbo of the House. But this particular passage applies with equal force to Reagan's deep frustration about international terrorism, which popped into public view at a news conference last June 18 while passengers on TWA Flight 847 were held hostage in Beirut.

"I've pounded a few walls myself when I'm alone about this," Reagan said then. "It is frustrating. But, as I say, you have to be able to pinpoint the enemy. You can't just start shooting without having someone in your gun sights."

Last week, U.S. planes had Libyan leader Muammar Qaddafi in their bomb sights and wound up killing his infant daughter. That is what happens in war, any war, and the provocations for this raid included many incidents in which terrorists directed or harbored by Qaddafi deliberately murdered civilians.

So, Reagan probably has a justifiable case that he acted in preemptive self-defense, even though he said at the June 18 news conference that "striking a blow in a general direction" would be "a terrorist act in itself and the killing and victimizing of innocent people." Information revealed in the wake of the U.S. attack suggests that the mission had the quite specific purposes of killing Qaddafi or, failing that, of encouraging a coup that would overthrow him.

But to say Reagan's decision is justified is not to determine that it is also wise. This judgment depends on intangible determinations not easily quantified. Will the raid deter future terrorist strikes? Maybe and maybe not, say U.S. officials, who are braced for future terrorism and additional retaliation. Would there be a similiar response to terrorism traced to Iran and Syria? Probably not, officials acknowledge, because there are significant differences in the consequences of striking these countries.

The most important intangible is the question of whether the raid had a long-term purpose beyond venting the emotional frustration building in this country since Americans were held hostage in Iran. One U.S. policy analyst who supported the mission worries that the prospects "after Qaddafi" may not necessarily be an improvement. He believes that Qaddafi may be succeeded by a less fanatic but equally anti-American military government more firmly allied with the Soviet Union.

If so, was the raid worth it? The policy analyst who supported the decision on grounds of self-defense has his doubts. "What's worrisome is that we don't have a very long look into the future," he told me. "We know where we want to be next week but not where we want to be six months from now."

Nor does the administration seem to know where it wants to be six months from now in its Soviet relations. There are signs that the Soviets, under economic pressures enhanced by tumbling oil prices, are eager for an arms-control agreement that would reduce weapons costs and inspire trade with the West. If Reagan is similarly inclined, he has given no clue. While few would mourn the loss of another happy-talk summit, a reduction of nuclear weapons would be valued long after the raid on Libya is a distant memory.

In this context, Reagan's recognition that time is running out on his presidency could prove significant. His retaliation against Libya has proved tremendously popular, but his time to negotiate with the Soviets is growing short. It is devoutly hoped that he remembers his insight of March 16, when he told the American people that he has only three years left "to work for peace."

Reaganism of the Week: Speaking to attorneys on Law Day last Wednesday, the president was greeted by applause and said: "Thank you very much. You make me feel very good in case Mr. Qaddafi brings legal action against me."