Senate Democrats negotiated a truce yesterday in their war over federal judgeships as Republicans agreed to allow more time to investigate President Reagan's nominees.

Under the agreement, the Senate Judiciary Committee will wait at least three weeks before holding hearings on new nominees and allow as long as two weeks before a committee vote. In addition, no time limits will be set for considering nominees that Democrats single out as most controversial.

The pact will limit the committee to considering six nominees at any hearing or vote and will add Democratic questions to the panel's questionnaire for potential judges. It also provides for the release of nominees' financial-disclosure statements.

Committee Democrats, saying Republicans were trying to rush nominees through, have delayed the process for weeks through parliamentary maneuvers. Some Democrats said they considered yesterday's agreement a major victory. It was negotiated by Chairman Strom Thurmond (R-S.C.) and ranking Democrat Joseph R. Biden Jr. (Del.) and approved by Senate leaders of both parties.

"Both sides wanted to get the deadlock over with," said Mark Goodin, Thurmond's spokesman. "They had to give up a little bit, and we had to give up a little bit."

Thurmond also said yesterday that the committee will vote next Thursday on the 17-month-old nomination of CIA general counsel Stanley Sporkin to U.S. District Court here. He also promised to try to bring the nomination to the Senate floor before Christmas.

Democrats had expressed concern that continuing closed hearings on the nomination by Sen. Jeremiah Denton (R-Ala.), Sporkin's chief opponent, would kill the nomination for this session of Congress. A fifth and final closed hearing on Sporkin is scheduled for next week.