If a soldier was told they were being flagged for an adverse action, how long does the command have to flag the soldier?

It should be initiated immediately, when it is actually in place depends on many variables. By no means would a tardy submission warrant the flag as invalid. The Service member would not have to concern themselves with timing if they stayed out of trouble. Thank you for your service.

Two working days a counseling should be issued after initiating the flag. Bear in mind that someone going over two days does not in any way invalidate the flag, it’s just a bit more of a process to go through but it can still easily be done... AR 600-8-2, PAR 2-5 should put it in black in white for you a little better than I can.

All depends. How was the soldier 'told' they were being flagged? Was it verbal or was it in writing? A verbal means literally nothing wheres a written counseling that due to said adverse event they being flagged means that the flag is either already in place or will be in place within a short window of time, the paperwork is in. Usually this is 2 working days.