Originally Posted by
Widgets
While you can be added to the airport standby list at any time, the gate agent isn't required to respect your position on the list if you are added after the check-in cutoff time. The gate agent can/should clear you last if you are added after the check-in cutoff and standby customers are already being cleared.
What I got when I asked about 25 minutes before departure was "Standby list is closed, go to your original gate".