So I’m a junior, and I want to apply as early as I can in the fall. Thinking I was clever, I went ahead and filled out a majority of the common app a few months ago. However, recently I learned that in the summer, Common App erases the information. When do they do this? Is it some random day in the summer? Is it July 31st? The latte would make sense, but I want to get everything in early so hopefully its the former 
in august
What I did was write everything out on a separate document and copy+paste into the Common App.
August 1.
Thank you!
and I might do that for some of the longer responses
You should always do that as there is no grammar check on the form.
Though the commonapp essays usually remain the same, I do believe that colleges change their prompts more frequently, so it might be in your best interest to wait until they put up questions specifically for next year.
Also remember that you may have more to write about at the end of summer or during the fall of your senior year.
Instead of writing the essays or personal statements as a whole, I suggest making a bulleted draft of what you would be willing to include in your applications when it’s time to actually start them. This’ll make it easier for you to type out your essays and even add anything new that comes to mind.