Hello Lorien, I have done some research, and everything (including this link below) says that it is hard coded and very difficult to change. Trying To Set Default Snooze Time | TheDailyReviewer
I did however have some luck looking in the registry, I found a key located here:
which seems to be what you are looking for, but I don't want to suggest adding or modifying registry keys that I can't ensure wont cause a problem. I have read somewhere that for Office 2010, you can change it by modifying this:
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\O utlook\Options\Calendar - RemindDefault
and the value is the number of minutes. If you are going to make changes to the registry, make a system restore point and back up your registry first.
Sorry I wasn't of any more help,