I've had it on for at least a year. 1 hour on, 2 hours off. It usefully slows me down when I get into a "someone is wrong on the Internet" mood.
I also highly recommend Leechblock [1] for Firefox. On my work machine, I have it set to give me an N second delay when visiting a long list of fun sites. As I watch the N seconds tick down, that's enough time for me to ask myself, "Do I really need to look at Facebook now?" Interestingly, N appears to be somewhat variable depending on circumstance. 10 seconds is enough normally, but when I'm not getting enough exercise or am dealing with work I don't want to do, N goes higher.
I also am currently loving Kanban Flow [2]. It's a Kanban board that has a built-in Pomodoro timer. (Briefly, the Pomodoro method involves working in 30-minute blocks with a mandatory 5-minute break. At the end it asks you if you've been focused; if you are, you get credit for the time.
[1] https://addons.mozilla.org/en-US/firefox/addon/leechblock/ [2] http://kanbanflow.com/
