In short: as thin as possible, and as thick as necessary, with the thickness behind the edge being more relevant to performance than spine thickness.
When I was first forming my knife habit, I gravitated toward thicker blade stock, succumbing to the "more=better" fallacy.
Over time, however, I realized that my use almost never justified anything thicker than about .125"/3mm, and my Opinel was (and still is) my best pure cutting tool.
Some uses certainly are better served by thicker blade stock, but these tend to be secondary to the main function of a knife, which is to cut.
Appropriate thickness for a blade depends greatly on the grind. A good example is the Mora Knife:
I loved my Companion so much, that when they came out with the Heavy Duty I just had to have it. Why not? it was the same, but more, therefore better!
Not in this case. Scandi grinds, it turns out, do not lend themselves well to thick stock.
Which makes perfect sense: The Scandi grind is basically a wedge, and with the grind angles the same (about 24deg inclusive) on both Companions,
the thicker stock produces a wider bevel. Which means more contact with the medium, more friction, more deformation and wandering of the cut.
God forbid you try to slice salami with a Companion HD.
This is why I'm very interested in the new Kansbol, and not at all in the Garberg.
On a folder, anything over .125" starts to be a turn-off for me, unless it's a very high flat grind.
Now, as for my Heavy Duty Choppers...
2mm at the handle...tapering to 1mm at the tip... over the space of about 14"-18"...