I try to only buy new tires when the old ones are worn or otherwise need replacement. This keeps costs down and encourages me to ride more, if only to wear out tires I don't like... or am 'tired' of.

Ironically, I find the tired which are the most pleasurable to ride are lighter and don't last as long. The heavy, thick and slow tires that I like less last much longer.

My current Specialized 'Blackbelt' tires have lasted for about 5000 km and 2,5 years. I can't wait to get these slugs off my wheels. They replaced a set of Panaracer Paselas (the ones without flat protective layers, TG or PT) which made the bike feel lighter and faster, but got too many punctures and needed replacement after about 2000 km.