I was a co-founder of 8aweek.com (YCW08) which was a similar concept for firefox.

See screen shots: https://addons.mozilla.org/en-US/firefox/addon/6562/

The trouble about doing it for one browser is that your other browsers quickly come into use.

One system wide fix is to edit your hosts file: http://thingsilearned.com/2007/05/04/combating-internet-addi...

but it only blocks you from sites instead of giving you nice time limits. And it becomes too easy to change back.

I wish there was something that offered the limit functionality but for all browsers/access but currently i know of nothing.

RescueTime (YC W08)'s Get Focused app is the best implementation of this I've seen.