1mm Well, I should say, depends! SLA printers, quite thin... FDM printers, approximately 1mm. Typically you can count on 2 walls... you can crowd the the slicer to try to get a little less. On my printers, I can design to 0.9mm and when I move it to another printer, it places voids there. That means even if I tell the printer 0.9mm, it actually comes out to 1mm or so.

targeted test prints are great for characterizing particular printers.