Have a look at K9 Web Protection. It is a child's Internet blocking tool, but have used myself with great success. You can block for certain amounts of time, certain days of the week, all the time, and can any time override it with a password or enter a mode that has no restrictions (with the password of course).
It's free. Highly recommend!
More info from another answer of mine to a different question:
K9 Web Protection is a child's (or children's) web filtering and blocking software that is extremely configurable. It offers: web-page filtering, word block, site block, category block, and the Internet time feature. You can set what hours of the day you want the Internet on the computer to be blocked, and can customize each day of the week. All settings are accessed via the web browser, and are protected by a password, which you could give to someone else and not know yourself if you wanted to completely block access. It works cross-browsers; one configuration covers Internet Explorer, Firefox, and others all one the same machine.
A big point is that you can turn off different aspects of the program, like turn off the webpage filtering, and just use the time feature, or whatever...
Absolutely no crapware or trialware or anything, just plain free! all you have to do is give them your email for a product key, and it works...they don't send any spam, or bug you about anything...