20" wheel tires get more wear than bigger 26"+ wheels. Contact area is limited to a smaller area, due to the small circumference. Over the years I have noticed that it takes about 6 months of daily riding to wear out a rear tire. Once it's worn I get flats on a daily basis. Front tires normally take me twice as long to wear out. My results are correlated to my weight and wild riding style. Lighter more patient riders should be able to go for longer.