How can I tell how much space has been used per month?

You can check your account usage by navigating to your Account->Usage page. 

Here's the direct link: https://www.jotform.com/myaccount/usage 

Or you can simply click the human icon at the top right most corner of the page, you should also see a quick summary of your usage:

Please note that monthly limits reset every 1st day of the month (EST).

Let us know if you have additional questions.