You know, I've never really given it too much thought. You are right though, there are a lot of variables to consider. One big variable is the cut of the tobacco, and how you prepare it. You can have a big difference in the amount you use per bowl just between fold and stuff, and fully rubbed out for a flake for instance.
Regular flakes ( such as Erinmore ) ..... about 15 bowls. Irish Flake at least 30 ( half flake a bowl ). Mixtures about 20 bowls per, but then again my biggest pipe that I actually smoke a full bowl out of is a Peterson Aran 03 ( would that be a "medium" bowl ? )
Per 50 g tin ? They usually last about 8 days ( unless, of course, it's Irish Flake ..... or 1792 )
The regulation portion at the Slow Smoke Contests is 3 grams. The contest pipes appear to be about a Group 4. So 17 bowls per 50g tin. 2 oz tins would be another couple of bowls. With flakes, I end up with longer duration smokes but fewer bowls, since I don't do the rub-out thang.