Treadwear ratings on the tire will give you an idea of how the tire will wear compared to standard "100" rated tire - "200" means it will wear twice as fast. Actual wear will be determined as the folks mentioned above.
One item to note is that tire wear is not linear. A new tire with deeper tread will wear faster than older one with partial tread depth. Wearing half the useable tread away in 5000 miles does NOT necessarily mean the tire is only going to last 5000 more. (Extrapolation will get you every time.
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