Oooo...where are the timer settings located? The only reason I leave mine on 24/7 is I like to sit down to a booted and working PC with my first cup of java in the morning. If I could have it come on a few minutes before I usually sit down to it that would be ideal.
Oh, but then again....I have my virus scanning happen while I'm sleeping. Hmmm....
Anyway, by leaving it on all the time I think it gets dustier a lot quicker because it's always pulling air into it. Unless you live in a totally dust-free environment I think that may factor in to the longevity of the components over time but I never tested that theory.