It varies, but a lot of it is 9 or 10 mm, roughly 0.35 to 0.39in. Reference: A company that sells Duck Tape. read more
Cheap, and I mean crap duct tape usually tops at 6 mil. Run of the tool box HVAC grade is 9 mm. Heavy is 11 or 12 and some very heavy get to 14. Things like Gorilla Tape I believe are 17 mils but these are really a glorified retail item. read more
Long story short, a good general use tape should be about 11 milli-inches thick, use a natural rubber-based adhesive, and be made using a co-extrusion process. read more