I have left mine running for months, never any problem except cooling fans.
Solid state parts can be left in a "power on" mode for literally tens of years before any significant degradation may occur.
Mechanical parts, on the other hand, like hard drives and fans, which have moving parts relying on bushings and bearings, have a definite lifespan that correlates directly to hours in use. If you leave your PC on all the time, these are the parts, especially fans, that commonly can, and do fail in a time period ranging in months instead of decades.