I might be able to shed some insight to the number of minutes used as I candle all 200+ cell phones for my company.
My typicall user is under 1500 minutes a month. This includes nights, weekends, and personal use.
I have a few users that peak over 3000 minutes a month, but their job demands that they be in the field and on teh phone almost all the time.
I myself, typically use less than 1000 minutes a month as I usually handle everything via email.