Try stayfocused- I used it all through med school. Half an hour on my 'big 4' - smh.com.au, HN, facebook, wired and later reddit a week. During exams I'd hit the nuclear options n and only allow wikipedia and university sites.

Great productivity booster

Edit: whoops forgot the link. https://chrome.google.com/webstore/detail/stayfocusd/laankej...