When are new options contract dates issued?

So presently, IEF has options expiring in May and June, then not again until September, December, etc. I assume the calendar is determined by the level of activity on a particular option.

How can I figure out when July contracts will be issued? I'm interested not just in IEF, but generally how to determine when new dates become available? I want to mark my calendar so I can swap out the option for one further out in time once it becomes available.