I've left my MBP running under load for days on end (and the G4 powerbook I had before it, for that matter) without major problems (touch wood), but I'd be a bit wary if the machine is getting really hot. Thermal aging of components can be a problem in the long term if you let it overheat regularly (I have a lot of experience in this area as I used to be involved in the design of Class A audio amplification, which tends to run very hot indeed).
Apple don't publish any hard and fast rules that I'm aware of - I'd just suggest monitoring the temperature of the machine, ensuring that the screen and hard drive can sleep even if the machine itself doesn't, and don't run it that way in clamshell mode. If you can keep it somewhere cool-ish, so much the better. If it IS getting excessively warm, then you could well be reducing the life of the machine - it's one of the reasons why the MacPro has so many fans...