Checking with co-authors on status of a not-yet-submitted paper?
Publications plans can end up going into limbo for a long time.  In my experience, this can be especially true for publications with lots of authors, because you can easily end up in a situation where nobody's really stepping forward to get it written, or where everybody thinks somebody else is the one dealing with the current road-block.  One paper that I am an author on spent three years in un-submitted limbo for such reasons.

As such, checking in every few months is totally reasonable, as you may in fact provide the stimulus to keep it moving forward.  Be warned, however, that if it's ended up in limbo and if you actually care about the paper, that you may well find yourself one of the ones getting handed tasks for actually writing it.