I bought my used kart (Oct 2012) with lightly used YLCs and I used them for an additional 7 race / practice days. They were still giving good lap times (front row starts) when I decided to change them. It was our club’s big annual night race and I was looking for an edge–there was no real need to change them because my lap times had been the same.
When I switched to that new set (June 2013) I wasn’t any quicker. I set my personal best on that same set in October–6-7 races / practices later, and was within a tenth of my best on a cool December practice day. I used them for a practice day in February (at another track). In fact, that set is still on my kart, and I plan to practice with them in a couple of weeks.
My experience suggests that the tire is durable. I keep them in a climate controlled environment when I get home and deflate them when not in use–that’s it.