xgouchet

I think that depending on your background, experience and ambitions, your mileage may vary.

All in all, I think that less than a year is too short. Less than a year will mean that you may not have learnt enough at the position you were holding.

As for too long, I'd say 7 years. Starting at the same (or similar) position the same company will mean that you've become too specialised, especially in the way you do things and you'll have a hard time adapting to a new company, environment, team etc...

Of course these can vary if you're particularly active in side projects or communities that will increase your experience, and thus, value.