Good question. Edmunds assumes 15,000 miles a year for use in the various tools here, like the True Cost to Own tool. Lots of sites assume 12,000 miles a year. But the actual number varies widely.
I put 32,000 miles on my '99 van back in '99/'00 and some 13 years later, I'm up to 182,000 miles, which averages out to 13,000 miles a year. But my 16 year old '97 wagon only averages a bit less than 6,000 miles a year.
Some people would say my van has had a better "life" since it's been driven more. The wagon usually doesn't get run hard enough or long enough to warm it up good and that can cause its own set of problems, like excess moisture in the oil or exhaust system.
The more important factor in buying a used car is gaining access to the maintenance records and having your own mechanic give it a good pre-purchase inspection.