I've frequently read that a good rule of thumb for many vegetables is that plant starts are ready for transplanting into the ground once they've "developed their second set of true leaves." However I'm unclear as to whether this means "once the second set of true leaves begin to emerge" or "once the second set of true leaves have grown to full size" or something in between.

I usually let the weather dictate when I plant out, but this spring has been mild enough that I've found myself with a huge window of opportunity. When do you all know your starts are ready to go in the ground?