PC hardware is designed to operate without error for a very long time; months or more of continuous uptime would have very little effect on the computer itself. What is dangerous is the software which is often fundamentally flawed and will eventually crash.
That said, my laptop is virtually always on, and my consoles are always off. The consoles see very little use, and run especially poorly if left on for too long. (Being a computer science major in university, my laptop sees nonstop use.)