Depends on level of detail , and how the skin is used by the 3dg. If it is used half way down and mirrored it can take up to 8 hrs to get the position of flaps right...eg on my Bear skin it took hours just to get the position right you ghave to mod it ththen save it as ...I leave the psp7 open go into the sim instant action , and check the skin in flight!!
It can be time consuming ...
but to answer the question...once everything is in place like panel lines rivets etc i spend around 2-3 days total on a detailed skin and average 4-5 hours on each,...