It's a valid and interesting question, and I too am curious to know how long the professional builders - Mike Brookes, Allen Martin, Mark Wheland, etc., take to build a rifle.
Talking about scratch built rifles, that is from a plank, the fastest for me was a John Armstrong rifle, with carving, hand made pierced patch box and toe plate, engraving, that I built in six weeks, but I worked non stop without days-off averaging 14 hour/day.
It took me five and a half months to build the Kuntz rifle, again, working fairly steadily, but accomplishing some career work too.
For a "parts set" from TRS, which includes building the lock, I just did a Baker in exactly one month.
The most I have built in one year is five rifles, and some of them were from pre-carved wood..