Are you using Wifi? Mine sleeps whenever the screen is off as long as I'm not using Wifi. If it's on Wifi it barely sleeps. I haven't looked into it yet, I'm sure it's a setting on one of my apps that has it syncing often when on Wifi.
Some tips for reducing awake time:
disable as much background syncing as possible
individual apps often report usage and whatnot, disable those options
set Facebook to either Wifi only or manual sync
find an app that tells you which apps are issuing wakelocks
disabling the use of network time/date helped on my Photon, but I haven't needed to disable it on my GS2