CO allows same-day standby for free, or confirmed within 3 hours of the new flight for $25.
Note: to get the "free" standby, you need to go to the gate. If you ask a ticket agent at check-in, they're going to push the $25 option. I did do the $25 option once, I was trying to leave about 12 hours early... tried first to switch to a NW flight, found out that it was oversold and they were going to re-accommodate on the CO flight (which had 3 seats left). I called CO immediately and got one of the last few seats for $25. I don't believe I would have made the flight if I had done the "free" standby.