Simply run the
uptime command inside a terminal (eg. via
adb or through Terminal Emulator).
You will get an output telling you how long the device is running for since the last boot.
up time: 02:57:16, idle time 10:21:25, sleep time 00:00:000
up time: as you may have expected, it's the time since boot.
idle time: I currently don't know much about where this time comes from/how it is generated; it seems to be the time it was idling in the past 12 hours.
sleep time: seems to be broken,
00:00:000 is the time I get even though the phone was asleep most of the time.