There is no built in way to do this. It's not a feature of Gravity Forms. You will need to delete them from the database using some SQL and a cron job.
Here is a snippet of code which shows how to delete every entry for form 1 immediately after submission. Do not use this code. You want to use something like this, but combine it with the SQL to check the time stamp and determine if the entry is more than 90 days old. If it is, delete it. Again, don't use this code as is. I am posting it only so you can see what information needs to be deleted when you delete a lead directly from the database.
Read this code, but please do not use it as is:
Posted 8 years ago on Wednesday January 9, 2013 | Permalink