Depends what you use it for one of mine is used as my network server
so its on 24/7 thats partly because sometimes my other half goes on the net during the night if my little baby daughter has woke her up.
Yeah, thats not much to worry about. Obviously 24/7 for weeks on end is going to start making a serious difference in performance if you don't switch it off and on; and will be noticable on the electiricity bill. But it won't damage the computer seriously or noticably. I think a month would be the longest I would like to keep it on!
I leave mine on for days at a time, at the moment mines been on none stop from over a week. It dont waste that much electricity i dont think (buy you'll have to ask my dad for a more acurrate answer)