It means, basically, just until they are not shiny like warm, raw cookie dough anymore. I wouldn't go longer than 12 minutes for the first sheet, then see if you like the texture of those when cool. (Twelve to 14 minutes is quite a long time to bake a cookie at 350.)y'know, as easy as that recipe sounds, I'm still befuddled by "bake until cookies are set in center". What the heck does "set in center" mean?