When to stop using pull-ups overnight?

TinyPinkBug

Brianna has been completely dry overnight for at least a month now, we are still using pull-ups just in case but I hate to buy another bag when we keep tossing them dry.  She just seems young to expect to be dry overnight, even though she is dry.  I don't want to cause her stress by putting her in undies overnight.  Am I just over thinking it?  She has a gallon size bladder, when she pees, she pees, and pees, and pees.  I think her bladder extends into her leg or something, I just don't know how she does it.