I use Chrome Nanny to block Tumblr entirely in Chrome so that I have to open up a Firefox tab if I really want to use it. (This was only somewhat a good idea though because I cannot figure out how to make Tumblr Savior work over there once it's installed.) You can also use Nanny to limit rather than entirely restrict your time, and you can set it for different days, but it's a little bit tricky to figure out how its settings work.