How long do submissions stay in my account?

It should stay in your Submissions Page indefinitely. Just bear in mind that Free Plan users are limited to just 500 Submission Storage, tops.

This means that if you reached 500, the oldest submissions will automatically be deleted to give way to newer ones. Bottomline, you can only have 500 submissions at any given time. As of now, you only have 2 submissions so you're way beyond your limit.

You may also delete submissions to free up your Submission Storage space.

Complete guide: How-to-Delete-Form-Submission-Data

(Review the guide carefully since this process is irreversible)