There is a "theoretical" standby deadline but it seems not be enforced.

I believe that you are supposed to standby 30 minutes before flight departure

On a MR (ORD/MSP very cheap fare) I got off the plane and noticed that the next lane was boarding to ORD. I asked for standby and was granted it. I spent about a minute at MSP.

Returning from FRA, I noticed an IAD-ORD flight about to board. I hoofed it to the other end of the IAD D/C gets and was allowed on. I got home about 90 minutes earlier than I had planned.

When I get off a plane, I always check the board for my connection city. It does not always work and then you are only back to your original schedule.