It depends a bit on your local authorities, but most regions grant three types of plumbing licenses: 1)Sewer Tapper; 2) Journeyman Plumber; and, 3) Master Plumber. Sewer Tappers are the guys who hook up building sewers to the municipal sewer, and I don’t know too much about their training. They have a shorter apprenticeship than we do, and I’m glad they are around, since I don’t have the kind of big and expensive equipment they use to lay their pipes, and I’ve never been that excited about using a shovel…
Journeymen need to work about 4-5 years as apprentices, pass an exam and pay an annual license renewal fee. They usually work for a plumbing contracting company, but some are independent. In many locations, Journeymen can’t work independently because they are not allowed to get building plumbing permits. I would guess that in a place like NYC, the strictest rules would apply, which is that only Master Plumbers can take out plumbing permits.
Master Plumbers usually need about 3 years of experience as Journeymen, they have to pass an examination, pay an annual licensing fee, and satisfy other requirements of general contractors, especially posting a liability bond and paying Worker’s Compensation for their employees.
Hope this helps, and good luck to you.