Under severe conditions - whether you harsh winters or our scorching sun - two years may be all you can realistically expect a cover to last and still look decent.
Some people reject the new Weber covers for being too light weight, but I like that they breathe well and resist shrinking and hardening. I have a couple of the older ones I use to cover my unfinished projects. Totally stiff and pretty much don’t fit anymore. They crack a little worse every time I take them off.