If you are acne prone or would simply like to remove any residue from the brushes, try refreshing them daily with Bare Escentuals Quick Change.
For a once a week brush bath (some people wash their brushes once a month or every 2 weeks), try their Well Cared For.
If you are doing makeup as an artist (on someone else or professionally), your brushes should be washed after each use.
I agree that if you are using them for anyone else, they should be washed after every use.
for my personal brushes, I quick wash them with a fast drying spray after EVERY use. They work better when they are clean anyhow. I use the clinique makeup brush cleaner (it is pink!).
I deep clean them once a month or month and a half (i slack a little since i quick wash them after every use anyway) using gentle shampoo for the synthetic haired ones and a touch of conditioner added for the animal hair ones (though i hate those because they smell when wet).