The Maximum Duration my PC remained on would not more than 12 hours in holidays. I get only 4 hours the opportunity to stay at home and use my PC before I sleep at night .
Man, I leave my laptop on for weeks at a time... eventually it gets so laggy that I have to shut it down for a bit and let it rest.
my longest time is around 15 hours... but now it suffers it cant get on more than 3 hrs it gets switched off!
I had never powered my desktop beyond 8 ours at a stretch. If the desktop is not going to be used for another 30 minutes, I will usually shut it down...