How long Alert data is kept inside of Signl4 is dependent on the Plan in question.
The Start plan keeps data for 1 month
The Notify & Connect plan keeps data for 3 months
The Optimize plan keeps data for 6 months
and the Maximize plan keeps data for 12 months
If at any point you wish to terminate you subscription doing this would also delete the data on the Signl4 side for your subscription. Here is a quick article on terminating/deleting your Signl4 subscription: https://support.signl4.com/hc/en-us/articles/360015831057-How-can-I-delete-my-account-