I think fundamentally i don't agree with that.

Being a painter/woodworker/upholsterer is way less theoretical than being a street cop.

But if you hire an untrained one, or one that only learned on the job, you'll have a way, way worse end result than if you hire a formed one (or it will take trice the time and double the materials, if I listen to my father's stories). Two years is enough for low-level workers, i'd guess 3 to 4 should do the work for the police (considering it's 6 to 8 years and a masterwork for compagnons, pro woodworkers/upholsterers would still be better trained than police).