IMHO, since there is no exam, cert, etc.
A modern master bowyer would be determined by the best hunters/clients opinion of the quality of the work, and a certain level of reputation as a bowyer. I don't think that length of experience is as essential as quality of craftsmanship, durability, and uniqueness. In my business, many watchmakers and master horologists build unique timepieces and can repair or rebuild nearly any timepiece, antique or current.