+1 for what Ken said here. BTW,hello Ken. Its nice to hear from you again. I haven't seen any posts from you in awhile. CraigBernd lists badger hair as lasting ten years on his site. But I've read several posts of gents using brushes for 30+ years.
I'm sure there are several variables. Is it your only brush? Or do you (as many here do), rotate 2? 3? 25? Do you face lather or bowl lather? How do you dry your brush? Over aggressive use will shorten it's life by 10, maybe 20 years.
All that said, I would not be at all surprised to see a single brush user who took reasonable care of a quality badger brush have it last 30 years! (manufacturing defects cited above excepted, of course)