We try and renew the frame at least every year. A kart that is past it’s useful life will stop responding to chassis adjustments and feel the same all the time. That’s not to say it won’t be fast, I’ve seen guys do incredibly well on used equipment. You’ll just lose some of the adjustability and some of the durability perhaps.
A couple good examples would be Ashley Rogero winning and running up front on a national level on Tony Karts that are a couple years old. Also, Tommy Andersen running up front, winning one of the heats and setting fast lap in the toughest field in the country, TaG Senior at the Supernationals, on a Kosmic kart that had 2 seasons of national/regional racing on it. That, for me, completely blows the “karts go off after 2 weekends” myth out of the water.