Grading should be judged by the level of the work one produces. This art form cannot be counterfeited. If you do master work in all required fields you are a master in that field. When a football player gains 2000 years in a season he becomes an all pro. No body asks him if he has been playing long enough to be one. If one was to reverse the question it would be why has it taken so long for some to achieve the master level than others?
With today's tool technology and information access, a beginning engraver can reach an "average" level in record time. If or how soon one reaches master level depends on perseverance, talent, dedication, and a relentless desire to create exceptional designs executed with masterful precision.
Obviously some make it there faster than others. Those that can devote plenty of time to study and practice will get there faster than those with jobs, families, and obligations that consume precious spare time.