I'd have to say it's "vaguely defined" because it's not at all critical, and largely a matter of personal preference how thick you make 'em. In my fleet they might be 3/4" on some boats, or as thin as 1/4" on others. The boat itself tends to define for me how thick they shall end up being. Often they taper, where you have 3/4" at the tip, and 1/4" where it blends into the keel line.

In all cases I rip the stem lamination strips to the maximum thickness I can cold bend without cracking, and this varies from wood to wood, and from board to board. They might be ~1/4" thick on a shallow sweeping curve like the Outer Island, or as thin as ~1/8" for something tighter, like the Merlin canoe. As you see in the Merlin photo below, I don't mind using a heat gun to help make the bend, but a steam box seems to be a little more than I can get off my dead butt and tackle.

Ref: http://mysite.verizon.net/ngc704/merlin/Page%202.htm