That's not something that you can do out of the box with Windows Server. You'll need some sort of proxy software that allows you to configure time periods where access is and is not allowed, and it will probably need to be granular enough that it can grant access by individual users. Most of the web proxy/filtering applications out there can do this.
Alternatively, if you use such a solution to block the "naughty sites" from the word go, you don't have to worry about blocking access for 45 minutes each morning.
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CompTIA A+, Network+, Server+, Security+
MCTS:Windows 7
MCTS:Hyper-V
MCTS:System Center Virtual Machine Manager
MCSE:Security 2003
MCITP:Enterprise Administrator