My take on the subject is that if the brush starts feeling a bit hard or stiff to bend or use on the face, I will then set it fully into a solution of half cider vinegar and water for a couple three hours and slosh it around every time that I think of it during that time.
rinse well and dry for the next use. the first lather may be a bit puny but it will get better over a couple days.
I would only need to do this maybe a couple times a year at most. Once in six months on my boar so far.
This also makes a good solution for the cleaning of old brushes, especially in an ultrasonic cleaner.
respectfully
~Richard
PS. I have heard of folks using hair revitalizer shampoo when their brush gets a bit dried out.