Time estimates are educated guesses, based maybe on a small number of people building early models, or someties even pure guesses. My own experience, I have had some that took longer, some right on, and some a lot quicker. I once assembled a Royal 40L that took almost twice as long as the 10 - 12 hours specified. The GP Modern Stick called out the same time, and I got it done in about 9. I had read that the Goldberg Skylark required about 14 hours, and that's about what it took.

How much time the assembly takes will most likely depend on the situation of the individual building it.