I made a chrome extension to help you time yourself.
You can find it at:
GitHub: https://github.com/samarthtambad/leetcode-timer
Chrome web store: https://chrome.google.com/webstore/detail/leetcode-timer/hihcjkhhlbmckhhnjamfomegbnlffcni

You can set the time for each difficulty level. It automatically picks the time for the corresponding difficulty. Also, the timer is embedded inside the page itself so that you don't need to navigate/look some place else. Hope it helps spike your productivity.