How long can a newly married soldier reside in the barracks?

It all depends on post policy. The usual time is 14 days following marriage to be cleared from the barracks.

I was a Bn barracks manager for 4 years. They can't get BAh until they move out. When they clears their barracks room, the barrack nco is supposed to give an eMH prefilled out memo that gives what day they esch cleared the barracks. The espective barracks NCO need to double check that they are already in eMH (enterprise Military Housing - web based software tracking who is in which room which use is required by Installation Management Command). They then put those memos in their BAH packet. This is all provided they are stationed within 50 miles of each other. If more than 50 miles apart, then they each are still entitled to a barracks room but no BAH under the Army Married Coupkes Program. Your installation single soldier housing should he able to pinpoint the regulation or point you in the right direction

There may he some barracks space issues if tje command is trying to push someone out. Im not aware of any reg that specifically states how long one could reside in the bs after marriage. Regardless, that new couple needs to start house hunting.