I'm a PM of sorts, I don't do freelance but I do work for a small web agency. We're a fairly friendly bunch of people and don't fine or penalize clients for delays. That being said I'm the mean PM in the company.
This will all solely depend on your clients, their needs, and goals. Most of the time they have some sort of goal time frame for which they want to launch. You should offer them a visual schedule, and in that schedule allocate the time they have for feedback. I usually state 2 business days across the board. I also make it very clear at the initial kick off of the project that delays in feedback automatically extend the delivery date to of the final product. I also inform them I don't pad my schedule (I actually do, sometimes). It's also important to clearly communicate when feedback is due. I often include a mini chart, of when each feedback is due in each communication, again just to hammer the point. It would also be important to pull dates from the client of when they might have issues making those deadlines, ie we have a conference, board meeting, etc.
Internally this is a total issue for small companies or freelancers. I would add into your services contract that that delays of X% or value can result in a change order. Additionally it might result in a drop in resource availability for the project. Also - you might state that the project could be set to be delayed by your company and replaced with other projects while the delay occurs and will be subject to re-scheduling, of course this could totally fuck up your pipeline too. I would recommend termination, and cancellation, in particular if you bill against line items and you don't typically stay with in line item budgets.