Block sites at specific times
WasteNoTime is a Safari browser extension which should have the requested features according to the website:

WasteNoTime is a browser extension which is designed to help you manage your time spent on the Internet more efficiently.
Time Tracker feature gives you reports on what web sites you spent most of your time.
Instant Lockdown feature allows you to focus on your work for a period of time with limited Internet access.
Time Quota feature automatically block selected web sites when you have spent a preset amount of time on them each day.