If President Reagan agrees to a summit meeting with Soviet President Leonid I. Brezhnev, as the latter suggested in a letter to Reagan, State Department spokesmen say it is not likely to occur before late summer, while other officials suggest it may take place much later.

The question of whether and when there may be a Soviet-American summit has quickly become a matte rof great interest among U.s. allies, who would like to see the strains between Moscow and Washington eased.

Similarly, allied diplomats believe the Soviets urgently want such a meeting, probably for reasons ranging from a desire to head off a U.S. defense buildup, to improve their image around the world after the invasion of Afghanistan and threats to Poland, and possibly because they face serious economic problems at home and would like to curtail the arms race.

The Reagan administration, however, while not ruling out a summit at some point, has indicated it is in no hurry to join in one, and suggested that some sign of improvement in Soviet behavior around the world would be helpful in creating an atmosphere in which a summit could be arranged.

At the State Department, spokesman William Dyess said, for example, that "a Soviet withdrawal from Afghanistan would make our decision" on a summit "somewhat easier," though he added that the administration was not making that a precondition.

Meanwhile, some foreign diplomats believe that the fact that the Soviets have made such a proposal and that the White House is considering it may be itself contribute to an improved international atmosphere, at least for a while.

Yesterday, in a joint statement summarizing the main points of talks here between Secretary of State Alexander M. Haig Jr. and West German Foreign Minister Hans-Dietrich Genscher, it as reported that Genscher "expressed a positive view on the idea of a properly prepared U.S.-Soviet summit." But Genscher added that his comments were made "trusting that, until such a meeting will come about, no evens will occur that might impair its purpose."

German officials left no doubt that this referred to Bonn's warning to Moscow not to intervene in Poland.

The summary statement omitted any response by Haig to the summit question, but State Department officials said later the omission did not necessarily mean that the United States is opposed to the idea.

Questioned by reporters, Dyess said it was "a reasonable assumption" that a U.S.-Soviet summit could not be held before the United States and its major industrialized allies gather at an economic summit in Canada late in July. Dyess stressed that careful preparation and consultation with the allies was a rerequisite.

"We are opposed to having summit meetings just for the sake of summit meetings," he said. "We want to have some promise of concrete achievement."

The European allies are not only anxious to ease U.S.-Soviet tensions, they also are hoping that a summit will improve understanding between the two countries so that local disputes in Africa, the Middle East and elsewhere do not become East-West confrontation.

On the other hand, allied diplomats who have visited Washington recently have idicated that, while they want a summit, it is better to prepare the groundwork carefully even if it means delaying the meeting.

State Department officials yesterday acknowledged that the allied visitors' repeatedly expressed interest in seeing the United States and the Soviets resume negotiations on limiting deployment of medium-range nuclear-tipped missiles in Europe has "certainly expedited the process" of getting the administration to consider how to reopen those talks. The official said it was not a matter of administration reacting to great pressure from the allies but rather of being sensitive to them and taking their views into consideration.

The administration also wants to fend off any left-wing movement in Western Europe against deployment of new U.S. missiles on European soil, and talk about resumed arms control talks is expected to help in that respect.