I stopped playing for about 2.5 months and didn't miss it, but that's because it had gotten kind of boring. Our small casual guild was waiting for WotLK and we'd farmed Karazhan to death, and there wasn't enough enthusiasm for ZA. So I would just log in for daily quests and the occasional heroic or normal group run. And after a while I just stopped logging in. I cancelled my accounts and figured I'd wait until the expansion to come back. I wound up waiting until a month after that before I started logging in again.

As long as you are taking care of the things you need to take care of (health, job, school, family, etc) then you should be okay. If you find that time spent in WoW is taking the place of time you should be spending on those other things, that's when you want to step back and re-prioritize and/or set yourself a schedule.