So she had as many chances to call as she had sticks of bubblegum or until it was lost or taken away. That was good. It meant that if needed she could leave some sort of bread crumbs along the way. It might also mean that she could be tracked and the charm could be reversed, but maybe that was okay too.
"Is there anything else I can do? Other than like just wait around and then pull a stick of gum at the right time?" Lavender Brown needed some sense of agency in this situation and without it she just felt sort of helpless. And when she felt helpless, she started to cry and she when she started to cry - well, that was when the world started to fall apart in ways it couldn't always be put back together again.