For boar bristle brushes, people like to allow 48 hours so that the bristles can dry completely, down to the core of the brush near the handle. This is to promote the splitting of the tips, which is what makes a boar brush so soft and pleasant to use. The prevailing wisdom is that it is the wet to dry cycles that cause the splitting.

For badger, I don't recall having a brush that wasn't dry enough for me to use the next day. I have used 24mm knot brushes day after day to no ill effect. I think.