I try to clean my makeup brushes as often as I can think to. I can always tell when it's time to clean them when the bristles just don't feel as soft as they should. If you take care of your brushes -- they will last years and years!
There are tons of different ways to clean your makeup brushes. Just search it in pinterest and let yourself get overwhelmed (as pinterest does on basically everything for me). Olive oil, professional brush cleaner and white vinegar aside, baby shampoo is my tried and true.
Step One: Run your brushes under warm water.
Step Two: Pour a small amount (think 1/3 a penny) on the brush bristles and let the shampoo and water create some suds. Rinse several times.
Step Three: Fill a glass with warm water and pour some baby shampoo in the water. Stir around and let your brushes sit in the glass for 10 minutes or so.
Step Four: After that time is up, take all the brushes out of the glass and rinse thoroughly. Make sure there isn't any shampoo left in any of the brushes. Lie them on a towel to dry.
Then place your silky soft makeup brushes back where they belong. Easy breezy!