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).
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.