I've run it for well over a month straight, multiple times. Just ask anyone on my friend list.
I have both a fat 80gb and a slim that I mainly use now.
Nothing wrong with keeping them on for long amounts of time besides the power bill. It's actually worse to leave it on for periods of intense play only to turn it off as soon as you're done playing. Not allowing your hardware to cool down sufficiently is what causes small cracks in the chipset over time. So advice to those who don't like leaving their systems on: after you're done playing, or watching Netflix for a while, allow the PS3 to idle either on the home XMB screen or just the game or Netflix menu for about 15 minutes before turning it off for the day.