Doing mostly highway driving, my stock tires on my 21" Uberturbine wheels were at 8/32" tread depth after 4,800 miles. Starting depth was supposedly 10/32 according to tire rack, so call it 1/32 every 2,500 miles. Minimum legal tread depth is 2/32, so useable tread wear is 8/32. 8 x 2,500 = 20,000 miles. Many people don't like to let their tread get that shallow, so I can see reasonable people replacing them at 15,000 miles, especially if they do frequent 0-60 runs. I would probably replace them somewhere between 15,000 and 20,000 miles If I didn't plan on selling them along with perfect condition 21" Uberturbine wheels. Get them while they are hot people!
Keith