I've never had problems with carry-on's. On the few occasions I have been asked to put it on the belt at check-in so they can apply a tag - and I tend to be over 7kg as I have a hardcase trolley bag (which is mostly full) and my laptop bag attached to the hand rail of the trolley bag.

All in all, I am sure its about 11-13kg I tend to get away with every time but I know what to dispose of in the event of being challenged.