We've all been there.
We think we've done the work, but the client comes back with a change request. Fair enough. So you make the change and they come back with another. And another. And another.
Where do you draw the line?
How can you politely say enough is enough?
Ultimately, this is something you can only answer at the beginning of the project.
Firstly, you have to set expectations, set your boundaries. Make it clear how the project will be run, how change requests are dealt with. How your experience and expertise are going to guide the client to success.
Secondly, you need to understand the objectives of the project. The true objectives of the project - not the vanity objectives, but the business reasons.
Because, if the changes the client want are aligned with those objectives then it's vital to the project success that you deliver them. But if the request won't move you further to the objectives, or if you believe the client doesn't really know how to get to those objectives, then you can step in and say "as an expert in this field, this will not help you".
The key word being "expert".
Be the expert and show the client where the boundaries are drawn.