Your PS3 can stay on INDEFINITELY provided it is not allowed to overheat. ALL computers are best left on, provided you aren't overly worried about your electric bill. The act of starting up a computer sends surges through the circuitry which reduces lifespan, and as well the chips are made of silicon which will eventually develop microscopic fissures from the heating/cooling which comes from turning off and on. If left on, the fan on the PS3 keeps it a constant temperature IF the area which the PS3 is not too hot (i.e. near a heat-vent, or in a closed-up cabinet, or in an area that receives direct sunlight). Make sure the vents are kept clean, too, especially if you have pets or hate dusting, or both.
Many people (search this forum) never turn off any of their electronics, many others prefer to save energy. FYI, you can go to Wikipedia and look up "PS3 power consumption" and see what your machine eats, depending on the model (earlier models are vasty more power-hungry).
http://en.wikipedia.org/wiki/PlayStation_3_hardware
If you think about it, all mainframes, business computers, bank computers, etc. are never turned off.
My advice to you:
1. Keep your PS3 in a cool place.
2. Buy a nice surge protector (I use a Monster brand) to insure that the power is cleaner.
3. If you really want to be sure, purchase a battery backup/surge protector/power filter. This will not only give your PS3 the longest lifespan, but will as well protect you from killing yourself if your power goes out in the 23rd hour of LeMans.
