Do they wear out over years & years of use to the point of needing replaced? Is there such thing as too many hooks, splits, etc.... in the knot as to consider it worn out?
I’ve yet to replace any of mine. But I suppose if they’ve lost too many hairs or just don’t lather to your satisfaction anymore, it’s time to replace. But there is no exact time interval for replacement.
I"d guess a 2 or 3 in a life time. I use my brushes like a government mule, both boar and badger and they're holding up well. Honestly, I don't remember ever having to buy one out of necessity until I found this place anyway. If you'd like a new brush, treat yourself to a new one. The trick is not to go crazy, I haven't learned that trick however and that slops over to straight razors and soaps. Lifetime supply of both here.
I would say depending the quality of the brush; I'd say properly taken care of brush can last 20 years. I've personally seen a boar Made Rite last my Dad over 35 years, used 6 days of the week. Just a personal preference, I find the silver tip almost too soft, a Simpson best badger seems to last forever. That said, I'm sure a good quality silver tip serve you well for a good long time.
I've had brushes for six years and no sign of wear. I wouldn't worry about it, use and dry/store properly and you'll have a lifetime of use.
The only time you need to replace a badger brush is when you lose it in your travels. If you use and dry/store properly, you'll have a lifetime of use.