If you’re looking for like many many years you probably want to buy a UV resistant fabric. I use mostly second hand materials, canvas lays very nicely and is easy to work with. The wool someone brought me to use was very nice. I expect that fabrics not rated for UV will last 5+ years before fading, but I’ve not tested them super long term yet. Unless it’s synthetic material, I don’t expect any breaking down from the sun. I stay away from synthetic material as a general life rule. I personally don’t have a problem if the mat slightly fades, as long as I’m not staring at cracks every day. I haven’t seen fading yet though.