I find it helpful to establish a routine to wash to begin with, e.g. every Sunday. Then it becomes a regular chore and doesn’t get put off until I’m out of brushes! I recommend using a gentler soap for natural hair (avoid alcohol-based cleansers as much as possible, which does dry out brushes over time), like Dr. Bronners Liquid Soap (I dilute it with water).
For synthetic brushes, alcohol-based cleansers can be quite effective without the damage, so for foundation brushes, I will alternate every few washes and use an alcohol-based cleanser but normally find my normal soap + water mix to be sufficient! I like to take similarly-shaped brushes and swirl together, which is way faster than doing one at a time. I lay mine to dry on a towel and then flip them after a few hours.