Its not really practical, and theres time when it shouldn't be on, like in a lit. storm, or when your vaccuming, when your leaving on trip. I think my reccord uptime for my personal pc was around 6-8 weeks, for a server like 6 months(I tended to forget about it). That was all before the days of fan controlers.
Oh and its not really gonna hurt anything to leave it on, just know that the more its on the more dust will collect inside, and that will heat it up a bit.