Ah editing. Is there nothing less thrilling than having to go back to a piece that you already wrote and retool for the sake of a client? No really. But it comes with the territory of being a freelance writer.
Industry standard is to allow two rewrites included in the cost of the project. If you're charging an hourly rate, then you charge the rewrites using the standard hourly rate. If the amount of rewrites goes beyond two, then you should have a stipulation in your contract or Letter Of Agreement which states that there will be additional charges.
I've never really had this situation arise quite yet (knock on wood). But I'm sure it would result in client pushback. And nothing is worse than client pushback. It's a tenuous situation. On one side, you don't want to upset the client because after all you want to retain their work in the future. On the other side, you got to eat and pay rent. My advice is any client that will jerk you around and try to get you to work for free or make additional demands without promise of pay is not a client worth working for. It's tough to cut ties with a client, but in the end it is best for your sanity and yoru busienss. You might be able to come up with a compromise, such as doing the additional work at the standard rate, but you should be compensated no matter what.
We as freelancers need to uphold a certain level of self-respect. If we let our clients walk all over us, we are doing a disservice to the freelance community as a whole.