It seems that the holdups with FN were because of Colt not wanting to sell them the licensing rights, and FN dragging their feet in general. The Poles only took about a year (depending on what month they started making their own technical package) to produce a few prototype guns, and somewhat longer to produce workable guns. Still, if Germany decided to reverse-engineer some captured Meunier carbines to 8 mm Mauser starting in mid-1916 (when they were first used, and presumably first captured), how long would it be before they could start fielding rifles?
A good gunsmith hand building a few working tool room examples would probably take months, then it takes a few months to convert from say American Fine threads to UNF threads then building a production ready gun with all the thousands of measurements laid out would take a lot longer. Even drawing up Blueprints could take a team of experienced Draftsmen months. Then the manufacturer has to spend a few months making their first working production sample then the Army has to test it and it might have to go back to the factory because say the wrong steel was used for the Receiver and it failed after 3,000 rounds. Then there is a few months more whilst a production line is built or converted then there is a few months whilst the Army trains the men in its use.
Its probably much quicker to use the original weapon as an inspiration. Much like say the Sten which was knocked up and in production very quickly because the MP18 gave the designers a head start in knowing what worked, saving a lot of time re-inventing the wheel.
After WWII the British tried to build a working copy of the StG44 and failed miserably the Germans were masters of sheet metal stamping (and were less worried about the weapon lasting 20 years of service) something which British industry had a lot less experience of. It was found that simply getting a car manufacturer used to making body panels and getting them to make a stamping mould and banging out a receiver was not possible. This wasnt a unique British problem every country who tried stamped metal guns postwar failed, witness the problems the Soviets had with the early stamped AK47 recievers.