In TAO/tao/Transport_Connector.cpp is a @todo:
 @todo: This is not the right place for this
 // Purge connections (if necessary)
 tcm.purge ();

I "inherited" this todo entry.  It was already in the code when I did the transport_cache vs too-many-connections work (#2934 & 2935).  Even though I did extensive work in this area, I did NOT revisit the question of when to purge the transport cache.

In thinking about it now, however, I think that the right time to purge cache entries is when there are too many of them (obviously) which could be detected by keeping track of how long it's been since they were used -- probably using a incrementing count of calls to find_transport rather than attempting to track time-of-day.   This would be an enhancement to the cache to track this info, and a change so that the "should we purge now" check came when the cache was successful at finding an idle entry.

So this bugzilla entry is simply a place to discuss how transport cache purging should be implemented.