A long time, if your trainees are reporting to school without an experience-based conceptual framework of tools, processes, and mechanical principles. I showed up at A&P school with a pilot's license and a background of casual tinkering on lawn mowers, gokarts, and my SAAB ice race/autocross/hillclimb car, and found myself awash in a sea of lifelong motorheads with 10W30 in their veins, axle grease under their fingernails, and their heads full of tolerances and displacements. I could hold my own on the academic side of things, but in the shop, they all blew my doors off.Japanese society was short of backyard mechanics. How long to train mechanics? How long until they're good at it?
I taught a couple Iranian naval cadets to fly, (Pre-Khomeni) whose driving experience before they came to the US was limited to ox carts. They both learned eventually, but it was a long slow process, and one of them never really got good at it. He got shipped back home after the revolution and died in the Iran-Iraq war, while the other jumped ship, stayed here, got naturalized, and eventually got his commercial license. Where there's a will there's a way.