Here is how we recommend washing the brush:
As a general guideline, it's important to make sure that water is not allowed into the metal ferrule area at all times.
Start with running the brush hairs under warm water with the hairs pointing downwards at an angle to rinse off make-up. Do this for up to a minute, holding the brush downward at an angle, so that the water runs down the bristles rather than back along the handle. This should be done with the water only touching the brush hairs.
Next use a mild shampoo or liquid hand soap. Squirt a little on your fingers and apply it to the hairs. Gently squeeze the hairs, lightly massage it in, being careful not to scrub. Rinse and repeat until the water runs clear. Make sure when you are rinsing to keep the brush hairs pointing downward at an angle.
Run the brush hairs under hot water once again to rinse. Squeeze the bristles lightly between thumb and forefinger from the base to remove any excess water.
Brush it off a little on a clean paper towel and lay it to dry ON ITS SIDE on a paper or regular towel. Never leave a brush to dry brush hairs up in a brush holder.
Lastly, let the brush dry completely before using. As to washing frequency, once a week should usually be sufficient, though it might also depend on how "sticky" the foundation you use is - in some cases at least a very quick rinse after each use might be advised.