It depends where you are with your routine, but there should be a time when you are doing things maximally such as the hard 30 minute session. On your off days for maintenance the shorter sessions will be good to keep you from shrinking down and help you heal in the expanded state. For example when I'm coming off of a hard routine on my last day before rest I'll just do a medium session for a half an hour of just slow jelqs etc. In other words ask yourself how you are capitalizing on the expansion you have worked up to and how you are resting so you don't over do it.