I worked as a free-lance programmer 'on retainer' for a big project (I'd do work for them once a month or so). It's never too early to discuss what will happen when you want to quit. I didn't do that, and after being more and more bored with the work, less and less excited about the pay and the whole project, I still spent more time working for them than would have been deemed reasonable, because there was no way to cleanly break off this work relationship according to a predefined set of rules. It worked out OK in the end, but knowing that you can stop your free-lance work anytime you want (assuming you don't owe anything and the client doesn't owe you anything) and pursue other projects makes it a lot less stressful in some cases.