Freelance policies?

Whether you're freelance or have a corporation of one, and you work in something that requires modifications if necessary. What are your policies around the number of modifications and whether or not you charge for them?

I have a client that's beginning to get on my nerves, with the number of modifications he keeps asking for, even though each time I do exactly what he's told me.. long story short, this project is dragging on and I want to finish and move on to something else - I have a lot of other things to do.

I've never had this issue of seemingly endless changes, but I'm thinking of putting into place a penalty beyond 3 free rounds of modifications.

Freelance policies?
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