The question of how long the Belgrade conference reviewing East-West relations should last has now becomes the major sticking point blocking agreement at a preliminary meeting here.

Western and neutral countries at the 35-nation talks are united in opposing Soviet-inspired attempts to set a strict time limit for the main conference, which is due to begin in the fall. The object of the conference is to review the results of the 1975 Helsinki declaration, including controversial pledges on human rights.

"The Soviet bloc is now completely isolated on this issue," a senior neutral delegate commented. Of 28 countries that spoke on the question of the cut-off date in a closed session, only the Soviet Union and its five closest allies were reported in favor. The others were all strongly against the idea.

"Once we have agreed on duration, we can wrap everything else up within an hour," the delegate said.

However, Yuli Vorontsov, the chief Soviet negotiator, told neutral and Western delegates today that the Soviet Union intends to stick by it insistence on a firm cut off date. "Without a terminal date, there will be no meeting in the fall," he was quoted by those present as saying.

In private, Western delegates say that Soviet insistence on a cut-off date is designed to allow them to filibuster if debate on human rights becomes too detailed or embarrassing.

"With human rights on the agenda, the Soviets are determined not to get involved in long drawn-out negotiations of the kind which preceded the signing of the Helsinki declaration," a Western delegate said.

Unlike other obstacles at the present meeting, which has recently been making program after a slow start, the cut-off date does not lend itself to compromise. "Either you have one or you don't, was the comment of a senior neutral delegate.

"Under the neutrals' proposal, which is being taken as the basis for discussion, the conference would open Sept. 27, with Dec. 15 as a target closing date. The meeting would continue on a weekly basis if all work had not been completed by then.

So far, Romania is the only Warsaw Pact country to accept the formula. The others - the Soviet Union, Poland, Hungary, East Germany, Czechoslovaka and Bulgaria - have attacked it, invoking a clause in the Helsinki declaration saying the Belgrade review conference will be held "in 1977."

This phrase is being interpreted by the Soviet bloc as meaning that the meeting should not be allowed to continue beyone 11:59 p.m. on Dec. 31, 1977, a ploy ridiculed as linguistic trickery by other delegates.

Western delegations, including the United States, insist that they are not prepared to negotiate on the question of a cut-off date. The only solution, they say is for the Soviet Union to agree to the clearly expressed will of the majority.

The other, if less difficult, stumbling block is the role of specialized committees on such issues as human rights, economic cooperation, and European security.

While accepting the idea of committees in principle, the Soviet Union has been trying to limit the number of times they meet and play down their importance. Western countries want the main work of the conference carried out in committees to issues as detailed debate as possible.