Time the Meetings
It is difficult to qualitatively judge whether a stand-up is taking too long, especially if it only gradually increases in length.
Therefore
Time the Meetings and publish the results. Most of the time, attendees just don't realise the impact of Story Telling, not being prepared to Take It Offline, or not preparing have on how long the meeting will take. Make it quantifiable.
But
As with all measures, timing the meetings shouldn't be introduced unless there is an actual goal to accomplish due to a problem with energy levels. Once the goal is accomplished, the measurement should be dropped. Measuring for no particular reason leads to suspicion and metrics apathy.