If you are shaking the water out of it and the knot flies out and hits you in the forhead it's time for a new brush.
If after the first pass you look down in the sink and there is more hair than water, You need a new brush.
If you have to use a cloths pin on your nose to lather up because the brush smells so bad, It's time for a new 1.
If you see your wife using it to clean the edge of the toilet seat, definitely don't use it anymore. And get a new one.
If you happen to be looking around and see something you might like to try. GET IT. At least that's what your brain will tell you and you always listen because your so smart.
It is almost a requirement to own 1 of each type of knot. Boar in each band, badger in each grade. So don't even wonder about when you need a new brush. By the time you do you will have some back ups I bet . :<0)