Assuming you don't ride the gas/brake pedal at the same time often, try to power out of ditches, or go up steep inclines an average series 20-30 belt will last 6-8 months if the clutches are properly maintained. If you have a series 40/44 assembly then the belt may last up to 3 years, typically 1-2.
If you're crazy, don't clean the clutch, and love to drive out of ditches, you'll go through a belt a week.