I’ve seen it stated that you should clean your hairbrushes weekly, bi-weekly, or monthly; as such I took it to mean to your personal preference, so I’m going to start washing my hairbrushes monthly. Per the links below, washing your hairbrush seems really simple, and only takes a few basic household supplies. Some warm or hot water, and gentle shampoo or vinegar should do the trick.
Basically: get as much hair out of your brush as you can; wash your brush in warm or hot water with a pinch of gentle shampoo, or use vinegar (use a bowl or fill up your sink); for a deeper clean use an old toothbrush; remove leftover hair and debris; if you use vinegar, make sure to rinse the brush under water again; place the clean, wet brush on a towel with bristles facing down to air dry; and voila, clean brush!
How to Clean Your Hair Brush (The Beauty Department) | Beauty, Hairbrushes
How to Clean Hairbrushes in Vinegar (Live Strong) | Beauty, Hairbrushes
How to Clean a Hairbrush (Real Simple) | Beauty, Hairbrushes
How to Clean a Boar Bristle Brush (Live Strong) | Beauty, Hairbrushes