Oddly enough, I had just measured the wheel studs today. about 1 1/2" long, total length, and 3/8th dia.
I have 15 inch Rota mini lite type rims, they use the cone type lug nuts. I purchased the lug nuts from R D ent. I did not change from original studs, after reading your post, I put a hub up to a wheel, and looked at how much thread would be engaged. It appears about 1/4 - 3/8ths inch engagement. I'll try to give a more accurate reading tomorrow.