Design/ engineering are probably as important or more so than the "thickness" of the material used.
With that said, the industry standard appears to be 1/4- 3/8s inch if the skid is constructed from mild steel.
1/8 inch seems to be the standard for stainless.
A few are made out of aluminum and some of those advertize up to 1/2 inch thick!
The mild steel ones, are the ones I have had some personal experience with, and it takes quite a bit of abuse to tear up a 1/4 inch thick skidplate. While I have seen some bent up, none were breeched and better yet the skid prevented damage to the vitals.