Is there a rule of thumb as to how many firings can be expected from a 5.56 case? These are trimmed and sized with a small base die.

no. like a set of tires, it depends on how you drive and the conditions under which you drive.

quote:

These are . . . sized with a small base die.

Not very many compared to less extreme resizing. A small-base die, assuming you push the case in all the way until the shell holder bumps, will significantly work-harden the brass, thus shortening its life compared to what you would otherwise experience.