While working on Watchdogs Legion, we had to make a lot of weapons in a few time with my teammate Louis-Michel Lebeau.
Setting up everything, we had around 5 to 7 days to make a weapon from high poly to the last LOD in game.
We had to keep a "low" polycount since it was a 3rd person game and I like to think we did well :)
Here is the M249, around 8k triangles, made in 7 days. It was the max polycount/time we had for those kind of weapons.