Take a look at the links on this awesome blog (Zenhabits). This blog is all about simplifying ones life and achieving personal goals etc etc.
Great Software (windows and mac)
I think you would be most interested in the links found in the
--> Browser Plugins/Extensions <---
section of the link I posted.
Hopefully this helps you :)
I've tried lots of these timer programs. I even had my computer set to shut off at a particular time each night regardless of what I was doing. I thought this would be helpful, but after about a year I started to realize that it is far more important to develop self-control without these tools...at least in my experience.
It takes longer to condition yourself to only checking your favourite sites at certain times, but I think its a far more beneficial skill.
But what do I know? Maybe the tools in the link I posted are just what you need!
Let me know how they work out for you.