When the client's expectations become unreasonable and/or impossible to meet.
When the client's check bounces. :)
Similar reasons ... if we don't feel we can help them, we don't take the project. If we don't feel the effort will be worth the revenue, we don't take the project.
Our pricing scheme is different, beckie, in that we don't sell weekly or monthly maintenance plans. All maintenance is done and billed based on actual time.
But ... what we tell clients may be something you can use ... "We won't nickel-and-dime you by charging for every little 5-minute update, unless you nickel-and-dime us by sending lots of little 5-minute updates."