Data Point:

I was standing-by on AA1456 today (currently waiting on tarmac at DFW, ugh). It was a totally booked flight and I was #6/20 on the standby list.

They cleared #1 on the standby list at T-18 to departure. I cleared at T-13, and there were others that cleared after me. Not sure if the guy who cleared at T-18 was taking the seat of someone who they knew wouldn’t make the flight or what...

I rarely fly on “completely full flights” so I usually find myself clearing standby at T-45 so I don’t have much other anecdotal eveidence to share.