Heck, my day varies widely as it is. Most days I'm in at 8. I just stay until I get what needs to be done, done. That can fluctuate from 3 p.m. to 7 or 8 p.m. Anything over 40 is OT. Anything over 51 is double time. That OT is just as good as a year end bonus. It adds up.
It probably helps if you really love your job and look forward to going everyday. I'm one of those extremely few people who also view their job as their hobby. My co-worker recently switched to nights, so now I get the whole building to myself.