I replace my tires at approximately 50,000 miles which is usually in the 3-4 year range. I am sure that I could get more miles, but I prefer the ride and traction that the newer tires provide. I have had excellent luck with both Goodyear and Michelin tires but I also carry an air compressor and check my tires each morning before hitting the road.
By changing my tires at 50,000 miles I also am able to negotiate a decent trade in value that more than makes up for going another few years with the tires and keeping them past the point where they lose any trade in value.