I’ve used Self Control in the past, which I like because it’s a hard stop — once you set it, you can’t get back to the sites you’ve blocked until the time runs out, even if you reboot your computer. But it doesn’t have a way to do a daily schedule; you have to set it every time, I think.
I’ve also used Focus, which does have a schedule, and has a hardcore mode that doesn’t let you turn it off — but maybe rebooting will turn it back on again? I’m not positive — anyone know? If not, I’ll probably try setting up Focus on hardcore mode tomorrow, see what happens.
I just noticed they also have a password option, if you really don’t trust yourself — you can give the password to a friend. Might be useful for parental controls as well, esp. for kids struggling with focus on e-learning?