When you bookmark a message, it remains in your Bookmarks until it is manually removed. Bookmarks do not have an expiration date by design.
Symphony does, however, contain data purge functionality that will delete messages from a specified time period selected by the user. If the data purge is performed and bookmarked messages are within the specified time range, they will be deleted from your Bookmarks.
Refer to the Knowledge Article found here for further information on bookmarks.