How does the available space work for our account?

It says 100GB, is that per month we get an additional 100GB? Or overall data? If overall data, how would we clean up the old stuff?

Note ideally we want to keep all the form data overtime and not have to clean anything up or delete anything

The available upload space is not reset, the only way to clear it is by deleting the submissions where you received files, this guide will help you doing so: https://www.jotform.com/help/377-How-to-Delete-Form-Submission-Data 

You can also download your data prior downloading it from your account, these guides will help you with that: 

https://www.jotform.com/help/374-How-to-Export-All-of-Your-Data-at-Once

https://www.jotform.com/help/73-How-to-Download-Form-Submissions-as-Excel-CSV-PDF

https://www.jotform.com/help/356-How-to-Download-the-Submission-Files

If you would like to keep your data on your account then you would need to upgrade to a higher plan, you can get more details about our pricing and the account limits on this guide: https://www.jotform.com/help/408-Understanding-Your-Account-Usage-and-Limits