Though some go longer I will say that an ideal time to remove and reinstall braids is about two months. At that point your scalp would be ready for a proper cleaning and some deep moisturizing for the hair. If you must go longer try not to go past three months.
I also recommend that you trim or dust your ends upon each or at least every other removal. The hair tends to get dry after being braided up and so you want to catch these possible split ends before they become a real problem.