As of version 3.6.4, SSU stops caching links that has zen session id in it, so you should see the number of cached files remains stable.
However, if you do wish to clear your cache periodically, you can also do so manually via Admin Extras-SSU Manager or set a cron job on your server to have it done automatically for you.
The cron file can be found in cron/ssu.php inside the download package of version 3.6.4 or newer. You can use google or contacting your host for instruction to set up a cron job. This process is fairly quick and simple.
