Defense Secretary Caspar W. Weinberger must decide next month whether to endorse the "dense pack" basing plan for the MX missile, but technical experts and intelligence officials still differ on one crucial question: How long will it take Moscow to develop a counter to the new basing plan?

That would determine how long dense pack, at a cost of more than $25 billion, might be effective before either an expansion or an anti-missile defense system would be required to protect it.

Air Force Lt. Gen. Lawrence A. Skantze, the deputy chief of staff for research and development, said in an interview that "the difference of opinion" about the time it would take Moscow, at least theoretically, to react to dense pack and develop a threat against it "ranges from eight to 15 years."

That difference, he said, "is the major thing, in point of view, between some of the independent technical people who have looked at MX versus the perception of the people in the intelligence community."

In the view of technical specialists, if President Reagan makes a decision in favor of dense pack and asks Congress in December to support MX deployment based on this scheme, the Soviets could conceivably field a counter-weapon by late 1990 if they began work right away.

That would be only about four years after the Air Force's 1986 target for fielding the first of 100 MX intercontinental missiles in new super-hardened underground silos. Under current plans, it will take three years, until 1989, to complete that MX deployment.

If the intelligence community is right, however, it would likely be the mid-1990s before the Soviets could develop a weapon to knock out the MX/dense pack combination.

The independent technical specialists are members of a special panel appointed by Weinberger and headed by University of California physicist Charles H. Townes. For the most part, these panelists are scientists working outside government or former defense and military officials.

In their assessment, officials explain, the Townes committee members compared only the technical capabilities of the Soviet Union and the United States. The intelligence assessment by the CIA and other agencies took into account other factors, such as resource problems and inefficiencies in the Soviet system that could delay development of sophisticated new weapons.

According to a presidential directive, Weinberger is supposed to make his recommendation on MX basing to the White House by Nov. 1. The Air Force made its final presentation in favor of dense pack to Weinberger last month. Weinberger has kept his personal views to himself during previous deliberations about where to put the MX, and Air Force officers say they do not know his views now.

Within the Air Force and the White House, however, there is a general sense of optimism that dense pack will get a go-ahead, although there also is a feeling that Weinberger remains "uncomfortable" with the plan, as one official put it. One senior officer, not Skantze, said he believes "if Weinberger had his druthers, he'd kill it."

Dense pack, officially known as "closely spaced basing," would bunch the new MX missile silos close together. The Soviets would not be able to attack this formation effectively because the blast and radiation from their first attacking missiles would destroy or blow off course the following ones. The Air Force believes that 70 percent of MX missiles would survive.

The general optimism about a go-ahead rests on a feeling that the president personally wants MX, and because the Townes panel, officials said, reported that the existing Soviet missile force plus Moscow's projected force through this decade would not be enough to overcome dense pack. The Soviets, officials believe, do not have the precise "microsecond" control mechanisms that would be required to carry out a simultaneous all-out attack.

The administration has argued that the current American force of 1,000 Minuteman missiles, spread across the Northwest, is vulnerable to Soviet attack, and the Air Force says dense pack will create uncertainty for Soviet war planners.

A special subpanel of the Townes committee, however, said there were some things the Soviets eventually could do to overcome dense pack. Most bothersome, Air Force officers said, is the prospect that Moscow could develop "earth-pentrator" missile warheads that burrow into the ground after impact. The underground explosion could crush a nearby missile silo but avoid the blast that would knock more incoming warheads off course.

The next most troublesome, officials said, would be maneuverable missile warheads which could adjust their course for greater accuracy and thus use smaller atomic explosives that also would reduce effects on the follow-up Soviet strikes.

The Townes panel, according to Air Force officials, agreed that the dense pack plan had the capability to cope with these new threats -- if and when they are developed -- through expansion. While this might mean adding extra empty silos as a deceptive technique to fool the Soviets, or building new silos much deeper underground, it also could mean a costly network of defensive missiles to try to shoot down Soviet attackers.

Another reason for Air Force optimism about dense pack is a feeling that the plan should be more acceptable to Congress because it requires only about 15 square miles of land -- probably in the western United States--rather than the thousands of square miles for the "shell game" basing proposal of the Carter administration, which Reagan rejected.

If that assessment is correct, it means technical questions about whether dense pack will work will become crucial in Congress. The basic question of whether it is necessary at all also is likely to be raised by lawmakers who do not believe the Soviets ever could gamble on attacking the Minuteman force.

Because dense pack is so complex, military officials believe Congress will depend on testimony of the most respected technical authorities. Indeed, officials acknowledged they now are just as concerned about whether dense pack can be successfully attacked by American scientists as by the Soviets.

Two names seem to loom largest: Harold Brown, the nuclear physicist who was Carter's secretary of defense, and William Perry, who was Brown's chief scientist. Both men, whose positions currently are unclear, are highly regarded on Capitol Hill and what they say could be influential, especially on Democrats.