what happens when you run out of space?

I'm not sure if you're referring to storage space or submission storage space. If it is the first one, your form will be temporarily disabled until it's limit are reset on first of the month, or you upgrade. 

On the other hand, if you are talking about submissions storage, your oldest submissions will be deleted every time you are above provided 500 submissions for Free account, but you will be notified before that happens.

Hope it helps.

Let us know if you have further questions.

Thank you!