MY 4th Gen SS was at about 7000 km when she started to pick up power and milage improved markedly !! My 2007 "New Style" Silverado was at abotu 11,000 (yes 11 thousand) Km when it broke in. It depends on alot of variables. Where you engine parts fit tolerances were at, driving, actual tolerances on engine part fitment, type of gas, oil change , for example if you use synthetic from day one iy may never improve cause synthetic oil is so good at lubrication the rings just do not wear. I allways use regular oil for a couple of changes till I feel the break in.......belive me you feel it, it's liek you were just draggin ass for weeks or months cause you dont sleep well or whatever then all od a sudden you feel like a million !!!! You can feel the pick up in power and the milage improvements ! So if you dont have a feel for it you may never notice anything except better MPG ! In that case you have to keep good records or trust the DIC !!
Just enjoy the ride !