Actually, the tile issue was evident before STS-1. While the Enterprise was being prepared in Palmdale for transportation to the Cape, the tile installation was running behind. So in order to make the delivery milestone they used Styrofoam to fill in the gaps. Unfortunately during the transcontinental piggyback ride on the 747 the Styrofoam came loose and then the already installed tiles were also damaged. So at the Cape they not only had to install the tiles that were not installed at Palmdale but also had to remove and replace the damaged tiles. IIRC with all the preparation for tile installation and pull testing each tile 1 man-week per tile was required.