This is a very simple add-on, but it does exactly what I'm looking for. It logs the time you use on a site. If you remove focus to another window, it stops counting. When the limit is reached, it sends you to a polite "You've reached your daily limit for x". You can set times for each site individually, and mousing over the icon tells you what you've used and how much you have left. It is good for setting personal limitations. It does not limit the person who doesn't want to be limited, because getting into settings is super easy, adding and removing sites is also easy.

There is also a feature that puts the time stamp in the corner of the page. That works, but it puts that stamp on every page you have open, not just the ones you want to limit. I would set it to only display on sites that are being limited if I had the option, so that feature isn't perfect. But I just keep it off and mouse over the icon to check the time left.