Eugene V. Rostow, President Reagan's nominee to head the U.S. Arms Control and Disarmament Agency, estimated yesterday that it would take at least another nine months before the new administration would be in a position to enter negotiations with Moscow on reducing strategic nuclear weapons.

Rostow's comments at his confirmation hearings before the Senate Foreign Relations Committee provided a new public timetable, even though a preliminary one, for such talks. But the committee chairman, Sen. Charles H. Percy (R-Ill.), made it clear he felt nine months was too long to wait "for one of the most crucial and critical areas we have to deal with."

Percy cited an October, 1980, Reagan campaign statement that "as president, I will make immediate preparation for negotiations on a SALT III treaty," and said many people based their support of Reagan on that statement. U.S. allies also view the strategic arms talks as crucial, Percy said, and going part way into the second year of a Reagan presidency to even begin them was a long time to wait. "How long are we going to delay this process?" Percy asked.

Rostow said, however, that it was going to take nine months or so to reach a point where the United States knows "what we are trying to achieve . . . and what kind of proposals we want to make." Although the administration has had an interdepartmental arms control study group set up since February, Rostow told the panel that "as of this moment, I don't know anybody in this government, with whom I've talked, who knows what it is we want to negotiate about."

Rostow said that the issue is extremely complex, and that proposals made in haste and without a thorough intellectual basis could easily founder. Since taking office, the Reagan administration has made clear it was not especially eager to begin strategic arms talks with Moscow quickly because it wanted to get its big new defense budgets started through Congress first and make sure the Soviet Union understood that its actions in Afghanistan and around Poland were unacceptable to the Reagan administration.

Rostow, a Yale law professor and a Democrat who served as undersecretary of state in the Johnson administration, also laid out U.S. policy toward Moscow on arms and other issues in some of the most specific terms used thus far by the new administration.

Rostow called for a policy, to be developed in conjunction with U.S. allies, that would link arms limitation negotiations to "the effective revival of the Truman Doctrine" of containing the Soviet Union by "collective self-defense against aggression." It also would require Moscow to accept the rules of the United Nations Charter regarding international use of force.

Rostow suggested that proposals for very large arms cutbacks on both sides "might be feasible, perhaps starting with the largest missiles." The Reagan administration is known to be interested in new proposals involving big mutual missile reductions. Even though Carter administration proposals along those lines were dismissed by Moscow in 1977, Rostow said "no American administration could reject such a possibility out of hand" because "the world is becoming so unstable . . . that agreements which now seem hopelessly quixotic may well become practical politics."

In line with would-be reduction proposals, Rostow suggested that the well-known acronym SALT, which stands for Strategic Arms Limitation Talks, now become START, for Strategic Arms Reductions Talks.

To signal continued U.S. interest in arms control, Rostow also suggested that the administration would support the Threshold Test Ban Treaty and the treaty on use of underground nuclear explosions for peaceful purposes, negotiated with Moscow by the Nixon and Ford administrations respectively but never ratified by the Senate.

Despite Percy's description of arms control as a crucial item for Congress, only four of the committee's 17 members showed up to question Rostow. The others were Republican Charles McC. Mathias Jr. (Md.) and Democrats Alan Cranston (Calif.) and Claiborne Pell (R.I.).