Pretty simple - a through and extensive knowledge of all principles and materials used in the craft.
After 700 bows and 20 years I'd say I'm close haha.
but it isn't time behind the tools - it takes an inborn natural ability and affinity with the subject. You can't make a silk purse out of a sows ear.....
Here in England when the bow was all important it was a 7 year apprenticeship to become a bowyer.