A master grower would be one accomplished in every aspect of marijuana growing and be in full harmony with the plant. He/she would be an expert and experienced indoor and outdoor grower, hydroponic to organic and everything in between, be able to grow in a closet or at the largest commercial scale, understand the technical aspects of chemical fertigation and organics, have a full knowledge of botanical principals, have knowledge experience dealing with every harmful insect and disease that can harm the plant, complete knowledge of breeding, mastery of all processing and extraction techniques...
I would say one would need 10 years of experience to even think of claiming this title, for most it takes a lifetime. Most importantly when seeking to become a 'master grower' would be the understanding that you are far from becoming a 'master grower'.