That depends on if you're doing it for just one machine or a whole fleet.
For just one you can either open the BOINC manager and click through: Advanced > Preferences > Network Usage (tab) and then enter the Additional Work Buffer length in days

OR

Go to the WCG website and click through My Grid > Device Manager (it's in the left hand column) > Pick the device profile you want to play with > scroll down to Workunit Cache Settings and put what you want in there.

If you have a bunch of machines and you don't change the individual settings on any of them, then you want to use the second method.