Since version 4.2.0.0 the administrator has the ability to set the period after which old notifications will be deleted. In the default configuration, notifications are deleted after 30 days. The setting is done in the wp-config.php file by adding the constant PEEPSO_DELETE_NOTIFICATIONS_AFTER_DAYS as in the example below.

In the example above, notifications will be deleted after 5 days. You can set the value to 0 if you want the notifications not to be deleted. This article will tell you how to access the wp-config.php file.