I’ve had this happen to me before too. I actually have a regular client who does this all the time. I’m usually pretty lenient with him, as he keeps coming back. But If he keeps me waiting longer than a week, I’ll usually drop a hint like, ‘Hey, I wanted to make sure you didn’t forget this project is still in revisions. I need to know what your revisions are by [x date] otherwise, I’m not sure when I’ll be able to complete the project. My schedule will be pretty hectic after [x date] and I wanted to make you aware, so that you don’t end up waiting a long time for the revisions to be completed.’ That usually gives him the hint he needs to get his stuff together.
That approach doesn’t work with everyone though. For new clients who leave me hanging, they get two days. Then I tell them ‘I need to know what specific revisions you are requesting. I can’t leave this project in modification status, without knowing what modifications you need. Please give me that information by tomorrow, otherwise I will have to redeliver the project as is.’
A few times, I’ve gotten no response, and then I do redeliver as is, just as I told them I would. I’ve never had a problem. In fact, at least 3x, after doing this, the client disappeared without a trace and I never did have to complete any revisions.