I don't think I'm qualified to answer this thread but I'm as amazed as you are at the builds on here all I will say is it takes me between one week and six weeks to finish a build, that depends on the scale and detail of the kit.
I'm only at the basic skill level of painting and dry brushing so it doesn't take long to finish a build
I'm only at the basic skill level of painting and dry brushing so it doesn't take long to finish a build