Technically, you have 10 years after getting your initial license, since you can extend your initial license once by request. If you got a preliminary license first, you could delay the master's degree by as much as 15 years.

Edit: To be even more technical, if you go the route of Master Teacher certification through Pearson, you'll never need a master's degree... though I hope you like staying on the Bachelor's / Bachelor's +15 step 5 pay scale for eternity.