Just tried with lights on, cold, no practice. 43 seconds from starting to pop the takedown pin to pushing the takedown pin back in. But that’s with the spare bolt in hand.
Probably a good 13 seconds worth of fumbling with the charging handle not seating correctly when reinserting the BCG. I didn’t realize I pulled it back enough to come out of its track.
I would definitely not be able to be faster doing that during an engagement. I’d never try. That’s a task to do back at home, at your car, camp, or patrol base. Wherever your “sustainment” gear is for your situation.
Edit: retried, not cold, and got 30.3 seconds in the light. Had to use my knife to pop the cotter pin that time.