I put around 3200 miles on my 70cc ebay kit before the wrist pin wore out. It may be worth mentioning that I put all those miles on my bike in just over 3 months. I took the engine apart and found that everything looked like a new engine inside. Replaced the rings and have a new roller bearing to replace the bushing ($20 in total). I haven't reassembled the engine yet, but suspect that it will be as good as new.
Given that the brass bushings (in the bushed engines) is relatively soft, I would consider replacing the bushing every 2000 miles as a part of general maintenance...or you can just buy a new engine