I'd be suspicious of anything under a year. That doesn't mean I wouldn't hire, but I'd want to hear why you left so fast. Hell, I'd even accept "My last employer was bad and made me do X, Y, Z, and thus I left".
The longest job I ever had - 5 years... and I left because they changed overnight and culled all the decent staff - fulfilled my promises, got screwed over, left the next day. Before that, I don't think there was anything under a year but I was working freelance for a while and there not having a client with under a year of service means you're doing well. They just kept buying me back.
What worries me is not serial job-hoppers, it's people with unexplained gaps. It's also people who stay where they are forever (it's easy to know when you're onto too good a thing and just coast... and I've met plenty of coasters who never want to progress and, when they move on, they only have their way of doing things). Sure, again, if you can explain yourself and you come across as so passionate for that job that's probably the reason you stayed, but anything out of the average range needs an explanation.
For yourself? Always look at jobs. How else do you expect to know what the going rate is, what the growing trends are, where the industry is moving, what your competitors are up to, etc.? And every now and then one of those jobs you're casually browsing will seem so much more your kind of thing, and there's NO harm in just applying and seeing what happens. If and when you get the job, that's the time to weigh things up.
I work in independent schools (I'm not a teacher, I do the IT). Once in, as a teacher, your job is pretty much guaranteed for decades so long as you don't screw up. Would you like to know how many jobsites I pick up in my web filtering logs? People keep on top of what's happening, what the competitor schools are doing, where's not a good place to work (you could tell my old workplace was going downhill by the fact that they advertised for an Assistant Bursar, then another Bursar, then another Bursar three months later, etc.), how much you should be earning, what else is about.
Keep your ear to the ground. It helps if you need to leave. It helps with comparisons should be need to go and ask for pay-rises. It helps with knowing what's out there. And it doesn't take any time at all to do.
But time-limits? You leave when you have a reason to leave, and not before. Someone who leaves EVERY year? That's bound to make me wonder why.