I can’t speak for gridfinity bins in particular, but wall thickness can vary wildly depending on the purpose or look you’re aiming for. For medium-sized container (about the size of a closed fist), i would consider 15 quite thin and wonky (but strong enough), 25 a good, solid width, and 40 quite chunky.

You can just print a few walls using the slicer and see what feels good, which is what I’d actually recommend you do.