Currently, when our JIRA administrators add a new JIRA project, the JIRA plugin will not comment on issues in those new projects until Jenkins is restarted. Our Jenkins is a large instance with several hundred agents and many jobs, some of which take several days to run. Also, a restart takes close to 2 hours. So we don't want to have to restart just to pick up new JIRA projects.
I've looked at the source of JiraSite.java, and there is actually a comment at line 234 noting that the list of projects never expires, and suggesting the idea of having a timeout of perhaps 1 hour. I have a basic pull request to implement this feature, which I will submit shortly.