I don't think these are intended to last a long time, not as long as you own and use the AirPods. That's why there's an active market of third party replacements. In my brief look, it seems many offer whole sets (XS, S, M, L) despite if you only need one size; a few are thoughtful enough to supply a set in only one size, despite the fact you'll probably buy from them less often -- three pairs in one size will last you longer than one pair and three other pairs that you discard. (This ignores if you "cheat" and use near size pairs like I do; I start to feel discomfort with Mediums so switch to Small.)
You can make these last a bit longer if you clean them regularly with a diluted solution of gentle soap (Castile liquid soap is ideal), warm water, then dry off excess water and let them air dry. If you use your AirPods daily, having a spare set in the same size allows for this.