I've dealt with clients that eat up 90 percent of your time while only providing ten percent of the revenue. Never be afraid to fire the client.
This of course works best when you "can" say no (i.e. multiple clients and other sources of revenue)
I have clients right now that I won't be able to attend to for a few weeks or so. If that doesn't work for them, I'll help transition them to a situation that works for them. But likely they'll stay around since good help can be hard to find.