President Reagan acknowledged yesterday that he does not know how long U.S. Marines will have to stay in Beirut when they return to help maintain order in the war-torn city. The Marines sailed from Naples, Italy, last night.

"We have no way of guessing, nor will we speculate on what the time will be," Reagan told reporters in reference to his controversial decision to join France and Italy in reconstituting a multinational force to help maintain order in Beirut until Lebanese authorities can reassert control over the city.

The Pentagon said five Sixth Fleet ships carrying 1,800 Marines left Naples at 7:05 p.m. (EDT). Earlier in the day, Defense Secretary Caspar W. Weinberger said the Marines could be in Lebanon within 72 hours of embarkation. Only about 900 are expected to enter Beirut.

When he announced Monday that the Marines would return, Reagan said only that they were going for "a limited period." His decision has drawn a mixed reaction from Congress. Many members have expressed concern that the United States could become too deeply involved in the civil war whose tensions and hatreds caused the weekend massacre of Palestinian civilians by Lebanese Christian militiamen.

Similar concerns were expressed when 800 Marines joined French and Italian troops earlier this month to supervise evacuation of Palestine Liberation Organization guerrillas from Beirut.

In a meeting with Senate Foreign Relations committee members last night, Assistant Secretary of State Nicholas A. Veliotes found general support for sending Marines to Lebanon.

"But there was a lot of anguish in that room," Sen. Paul E. Tsongas (D-Mass.). "There's a sense that we're getting into a quagmire. We don't have a choice, but how are we going to get these troops out again, once they go in?"

Tsongas said that senators have received a flood of constituent reaction to the massacre and that the tone is "very strongly anti-Israeli." However, he added, any move to reduce military aid to Israel would be counterproductive because it might reinforce Israeli hard-liners as the United States is "looking to Israeli public opinion having a role to play."

Sen. Joseph R. Biden Jr. (D-Del.) said there is "a lot of talk in the corridors" about using foreign aid to Israel as a lever to force Israeli withdrawal from Beirut. Biden said he would take no position on Israeli involvement in the massacre until he knows the facts.

Although the administration originally gave the Marines the additional goal of helping the Lebanese army regain control of the city, Reagan bowed to the evident unease of Congress and the public and ordered the troops withdrawn when the PLO had left.

Despite Reagan's insistence that Israeli forces withdraw from West Beirut immediately, U.S. officials said yesterday that such a withdrawal is "not a precondition" of Marine redeployment in Beirut. The officials made that point after the Israeli Cabinet agreed yesterday to the return of the multinational force but gave no sign of when Israeli forces will leave Beirut.

State Department spokesman John Hughes said the U.S. position still calls for an immediate Israeli withdrawal. But he added that the Israeli Cabinet's move appeared to be a sign of willingness to cooperate in returning the multinational force to Beirut. On that basis, Hughes said, "We'd go ahead."

He noted that Israel appears to be thinning its forces in West Beirut, possibly in preparation for withdrawal. He also said it it too early to say how the Marines would be deployed when they reach Beirut. These details are being worked out in talks with the Lebanese, French and Italians, Hughes added.

Pentagon aides said the Marines, who were kept in the Beirut port area during their earlier service, probably will use the Beirut airport as their operations center this time. The officials said that while the Marines' job will be to help the Lebanese army take control, they will not engage in combat and will be withdrawn if major fighting breaks out.

Administration sources said privately that, while Reagan remains firm in his insistence that Israel leave Beirut, the U.S. aim is to achieve this goal through quiet diplomacy instead of issuing further public demands to Prime Minister Menachem Begin's government.

The sources said Reagan's tough statement last Saturday and subsequent U.S. communications had left the Israelis in no doubt about Washington's intentions on the withdrawal question.

Now, the sources added, Reagan's special envoy in Lebanon, Morris Draper, is talking quietly with the Israelis to develop a formula for Israeli withdrawal in advance of or simultaneous with the multinational force's arrival in Beirut.

Despite the Begin government's refusal to commit itself on withdrawal and despite hints from Jerusalem that Israel might seek to keep a military presence in East Beirut, the sources said they are confident that there will be no problems about an Israeli pullout.

Hughes, answering questions about who was to blame for the massacre, followed the administration's practice of refusing to reply directly. But his answer did contain a clear implication that the administration believes Israel must bear some of the responsibility.

"The fact is that Israel assumed responsibility for military control of Beirut, and these events did happen during that period when Israel assumed military control," he said. "If you assume military control of an area, you are responsible for what happens there."

Vice President Bush met for 1 1/2 hours in New York yesterday with 10 representatives of American Jewish organizations and heard expressions of unity from groups that in recent days have seemed divided about their approach to the turbulent events in the Middle East.

"When push comes to shove, there is unity when there is a threat to the state of Israel or its survival. Therefore assurances of the administration that there will be no economic pressure or other pressure to accept its terms is gratifying," Julius Berman, chairman of the Council of Presidents of Major Jewish Organizations, said Bush was told.

Berman added the group urged Bush to "avoid confrontation [with Israel] to the greatest extent possible. Confrontation doesn't do anybody any good, especially allies."