@Danny *********
you can apply for a new extension right up to the last day your stamp is good for IF you want to (although it's not recommended to cut it that close, in case you need another document, etc).
Most offices allow you to apply for a new extension when you have 30 days or less left on your current stamp (and you don't lose time going early because your next extension starts the day this one ends (so your expiration date is always the same date every year)