Big act? Well - REALLY big acts will be on license deals - in which case 50% or more..... but a big "record deal" would be 18 - 20%.
A crap deal would be 8% but they're not unheard of.
To be honest - whatever you can imagine will have happened at some stage - but 12% is about average for a first deal. Having said that - this is changing very fast and there is no way you could go to a label with a pop act and look for such a deal. They just do not exist.