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How much life you get out of a set of tires depends on a number of factors, some of which have been pointed out.
One thing that's less obvious is the quality of the rubber used when the tires were made. Rubber can vary significantly in quality from batch to batch, so while the design of the tire and grade of rubber can significantly affect tread life, the batch of rubber used throws in a big wild card.
I don't think tires on a new car come with any sort of warranty, but virtually all new aftermarket tires do regardless of whether you purchase a separate road hazard warranty (which is worthless as far as tread life goes anyway). This typically works on a pro-rated basis. So let's say the tires are rated at 30K miles and at 15K the tread depth is 3/32. The manufacturer will pro-rate them at 50% which means you get that much credit towards a new set of tires. Many of the manufacturers' warranties work that way, but you need to verify this at the time of purchase.
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