I turn mine on around 4 PM and off around 10PM Monday through Thursday. Friday through Sunday really depends what I'm doing, but overall I turn it off whenever I leave my room or come back. Even on my 'old' system, Williamette 1.7 and Diamond Max 8 hard drive, which is at the bottom of ratings at storagereview.com in terms of performance, it doesn't take much time to boot up. Sure, sometimes I want to watch TV and I hate waiting for it to boot up but its relatively fast and doesn't take much time. I don't usually leave mine on a lot just because it sits there idle using electricity.
I too need noise to fall asleep, I turn my room fan all the way up before I sleep. I've actually tried a setting or two lower and I couldn't sleep...