It is different allowances for different airlines but the best I have seen for carry on is six kilos European flights and ten for long haul although you can usually get a few extra in.
I must admit we do cheat when we go as a group, we book in as two smaller groups and the others wait with the heavy hand luggage and we all check in with the light stuff.
I think it is more the size of your carry on bag than the weight that gets the attention after you have passed the ticket desk.
Russ