What is the minimum thickness of skin required for Wikipedians who are not sysops?

You need a very thick skin, or in the alternative to not care what happens to your contributions. Since they have no problem slandering celebrities, it is no big deal to them to slander regular people, to make all sorts of absurd accusations.

Also, you have to understand that they don't care about your credentials, because even if they were able to confirm them they wouldn't care. So if you bother to add content to a topic you're an expert in, don't let it bother you when it all gets reverted for no good reason at all.

Practically no minimum thickness requirement. You just have to use common sense when editing or submitting information.

I'm thinking an average of slightly less than 0.05 inches (1.3 millimeters) in thickness would be sufficient for most people.

Not that it matters. You could edit Wikipedia without skin at all if you wanted.

But common sense is all the "skin" you need. And if you lack that, you can hang around on WR whining about how they "didn't treat you fairly" and how mean those big bad Wikipedians are. And people will happily leave you be.

All it takes is common sense helpfulness to do well as an editor. The biggest secret is to find information in a good source, then do your editing.