48 hours is fine. It is a little long, but 36 hours would be confusing and 24 hours would be too short. Maybe in the IM you send to the winner, you could put the date and time the winner needs to respond by? Like the current respond by time is 4:04AM 4/17/14. I do know that that would be extra work on your part, and that it is the winners responsibility to decline or acknowledge and respond on time.
First Jive Dude: Shiiiiit, maaaaan. That honky muf' be messin' mah old lady... got to be runnin' cold upside down his head, you know? Second Jive Dude: Hey home', I can dig it. Know ain't gonna lay no mo' big rap up on you, man! First Jive Dude: I say hey, sky... subba say I wan' see... Second Jive Dude: Uh-huh. First Jive Dude: ...pray to J I did the same-ol', same-ol'! Second Jive Dude: Hey... knock a self a pro, Slick! That gray matter backlot perform us DOWN, I take TCB-in', man! First Jive Dude: Hey, you know what they say: see a broad to get dat booty yak 'em... First Jive Dude, Second Jive Dude: ...leg 'er down a smack 'em yak 'em! First Jive Dude: COL' got to be! Y'know? Shiiiiit.