Piston skirt scuffing typically happens right away - on first fire-up. As mentioned above it can be due to dirt, a sharp bottom edge, or tight piston-bore clearance.
At Ford we did a lot of work with skirt coatings to eliminate scuffing with tight pistons. We were going tight to minimize NVH for future engines. Even though the coating would wear away quickly, it did its job in those critical first few seconds...