Cronenberg89 Take you working horse aka your mostly used brush with you to the art store and compare length of bristles to new one. If it is 1/2 inch shorter that brush has worked out much of its resource for application but can still live but for rougher works.
I mean 1/2" or 1" BR brushes - bristles abrade and we do not notice it most of the time. However brushes are made in the way that hair thickness varies from ferrule to the tip. Meaning as it serves as working tool the stiffness increases with decrease of length of the bristles and making some of the brushes more difficult to control in their normal applications.
Old brushes are still good for gesso, for creating things with character, like some foliage even even they became too fluffy, it can be used to create additional interest in the brush marks.