There are multiple factors that influence the wear and tear of a treadmill belt. The best way to know when it's time to consider a replacement is by doing a visual inspection.
Always check for curling, fraying, rips, and seam stability when you lubricate the belt every 300 miles or three months. Proper rigidity of the belt may be checked at this time as well as ensuring the belt is tightened properly.
Please see the resources below for more running belt information.