Currently from what I know, Ubercart uses a global reminder date for all roles. You will find this at admin/store/settings/products/edit/features under Role assignment settings (if you have the Ubercart Roles module enabled. This is in the standard Ubercart module).
In the Role assignment settings it has a Time before reminder section which is where you set the reminder.
In my opinion, this is one of the downfalls Ubercart currently has with its role handling. Most member sites you use these days give more than one reminder. You want to be able to give a reminder that has ample time for customers to arrange payment but then also give them one closer to the date in case they forgot.
Not sure if this is being looked at already by the developers or not.