I don"t know too much about other airlines but on UA. The upgrade is requested using 4 different instruments. With Miles, SWU, CR1's they clear as soon as there is availability. With e500's it clears based on status.
"e500s clear based on status. If upgrade space exists, upgrades clear according to [UA]:
* GS: 120 hours
* 1K: 100 hours
* 1P: 72 hours
* 2P: 48 hours
* 3P: 36 hours
* GM: 24 hours"
Quoted from
http://www.flyertalk.com/forum/unite...-upgrades.html