tangor - 5:35 am on Aug 3, 2010 (gmt 0)
In addition to above, I've successfully used limited time as well: 30days, 60days, 90, 120, etc... Some clients like that model and are willing to negotiate another block of time if they can't get it figured out...
On the other hand, if you want the client dependent upon your services (and they are okay with that) price accordingly.
All depends on the client, what they are capable of, and what they want as a result.