Men of college age may soon be drafted into active military service for six months instead of the current two-year periods, Assistant Secretary of Defense John A. Hannah said yesterday.

The shortened term of active duty, together with extended service in the reserves, is essential to fit the present national emergency, "when we are neither in all-out war nor at peace," Hannah declared. "Our military potential must be geared to our present day conceptions of warfare," he added.

Testifying before the Senate Armed Services Committee, Hannah revealed "radical and dynamic changes" proposed for the nation's defense manpower setup. The new program is near final Administration approval and will probably go before Congress within three weeks, he said.

As outlined by Hannah, the revised manpower system would include, first, a provision that draftable men between 18 and 26, who are now subject to two years' service, would take only six months of active duty. Thereafter, they would be required to attend regular drills and active training in reserve units, probably for seven and one half years.

Men who still complete two years or more of active service would, under the new program, be required to spend up to eight years as members of a standby reserve. Unlike members of the present inactive reserve, however, they would have to attend regular National Guard training during that period.

In the event of emergency, the government would presumably really men with six months of military duty before it would take two-year veterans from the standby reserve.

The crucial question Hannah refused to answer is how men drafted to serve actively for six months would be distinguished from those called to serve for two years. But according to last December's Adler manpower report, which seems to be the basis of Hannah's proposal, this problem would be solved by a nationwide lottery among the draftees.