SnoozeMaster solved this issue for me. It works with both Calendar and Reminders alerts, giving you many more options than the default OS X Mountain Lion 15-minute Snooze. I didn't want to switch to a non-Apple program (such as BusyCal) because I like the way Apple Calendar syncs easily with my other OS X and iOS devices. SnoozeMaster allows me to keep using Apple Calendar, but with a much more flexible Snooze option.
You can enter any value you like for the Snooze period. Alternately, you can configure your own drop down menu (similar to the default Apple drop down in OS X Lion). This configurable menu can contain as many items as you like, with time values of your choosing (5 min, 15 min, 3 hrs, 7 days, 0.5 min, etc.).
While running, SnoozeMaster lives in the menu bar under a ".zZ" icon. To insure that it's always monitoring your alerts, I recommend setting SnoozeMaster's preferences to:
Start at Login: On
I also recommend turning off Apple Notifications for Calendar and Reminders. These settings can be found in System Preferences:
System Preferences > Notifications > Calendar > Calendar Alert Style: None
System Preferences > Notifications > Reminders > Reminders Alert Style: None
Then you will only get alerts generated by SnoozeMaster and not Apple Notifications.