I usually have my 24" iMac "on" for weeks, if not months without problems. I just put it to sleep when I sleep. That means 15 hours on every single day. After a few weeks it might get a bit slower, but reboot always helps there.
Had mine for about 3 weeks but it is my personal/home business machine and never gets pushed to do anything CPU intensive. I have to put my ear next to the screen to hear the fan - occasionally I can hear the very slight sound of the HDD doing its thing if the room is dead quiet and I'm close to the screen. So far, so good.