My personal thoughts:
When you find yourself leaving games broken because you're tired of working on them. I left one broken for almost 3 years. I told myself no more until I got that fixed.
When your bank account says no.
When you run out of space and can't really convince yourself to sell/trade one because you know you'll regret it.
When you find yourself trying to decide if you wanna play or watch TV and you choose TV.