How many days is a newly wed soldier allowed to reside in barracks?

How long is your AIT?
If you get married in AIT and you are currently residing in the barracks and the AIT is under 6 months in length, you will be staying in the barracks until you get to your first duty station. Its not just as simple as getting married and receiving BAH.
You will need to enroll your spouse in DEERS if not dual-military, submit a DA 5960 to start BAH, Submit a DA 4187 to stop meal deductions, Provide the original copy of your marriage certificate to your S1 to go with the packet. Once all of this processes, your BAH will begin but if your AIT is less than 6 months, you will need to provide a copy of a lease for the physical location in which your spouse is residing and you will receive the BAH rate for their current location. Now if your AIT is longer than 6 months, you will need approval from the AIT Chain of Command to seek privatized housing which does not automatically get approved depending on what phase you are in currently and how long you have remaining. If you were married before arriving at AIT and had BAH already started, it would be easier.

Now lets say everything works out for you and you do get to move in with your spouse, when the BAH officially starts, you normally have no more than 72 hours to vacate, but can vary because you are in AIT.

Let me know if you have any other questions.

You're an activated Reservist? Have fun starting that BAH. You'll need a signed lease first to start the BAH.

If you are still in AIT, you will probably NOT be leaving the barracks.