I think for me some of mine have taken upwards of 40 hours. 20+ skin, and 10+ research.
The most recent one I worked on... I don't even have a count. I thought I was less than 10 hours away from completion, when I was informed that I was doing an invalid plane.
Now I think I've sunk 50 hours into it and still not complete because I keep getting frustrated with how it looks. lol