From other posts, I've seen people report 1-2 day usage out of an extended battery... Although, it'll take a couple charge cycles to unleash its full potential. That is if your extended battery is new.
If you see my thread, I am now having major problems with my TB. Battery will not charge above 88% and the extended battery is running out of juice quickly. Phone is unrooted and this is after a factory reset. Never had a problem until after the latest update.