Any decent badger brush should last years with proper care. Beyond 10 years is possible if proper care is taken. Rinse properly after use and allow to fully dry. Brush should be dried in an open space to prevent mold. I live in a town with hard water so I occasionally rinse my brushes in a dilute vinegar (water) solution. Remember to rinse well after the vinegar/water bath. Same is true for boar or horse hair.
There are conflicting arguments on hanging a brush to dry. Some manufacturers and enthusiasts suggest hanging a brush bristles down while others feel shaking the water off after rinsing and placing the brush handle down does a better job (wicking allows the water to travel up the bristle) I do both so really, I doubt it matters which, so long as you air them well while drying.
Most high end manufacturers stress that a circular lathering pattern is bad for bristle longevity, they warn you to "paint" the lather on. I don't, because I face lather and I like the exfoliation I get from a brisk circular face lathering. I also think that they are covering their behinds against any and all possibilities and are being a tad craven in the process.. Remember, in all areas of wetshaving YMMV (your mileage may vary) is the watch word.
I guess if the brush thins out, gets stinky, slimy or you no longer enjoy using it then chuck it out. So sayeth the Beas :)