I’ve had my 3090 since December 2020. It works great but I had to undervolt and slightly underclock it due to the insanely high hot spot temps, as high as 106 degrees under full load. This is a very common problem with 3090’s, or at least 3090’s at launch. I believe it had to do with inadequate thermal paste jobs primarily.
Aside from that, there’s no reason why they shouldn’t last several years. I suspect the 3090 will enjoy the same longevity that the 1080Ti has had, with many people still rocking them two or three years from now, even when the 5000 series cards are out.