How long is too long to wait for a rejection because of a lack of reviewers? I am posing this question here because of a mini-controversy that arose on another list on which I participate.

One poster suggested that the poster's "rights were trampled on," because a journal spent two months trying (but failing) to find reviewers for a particular paper. However, the journal did not notify the poster during that time that anything was amiss with the process.

At the end of the two months, the journal sent back a rejection notice, because they could not find peer reviewers. While I believe that the journal could have done a better job in notifying the author that the review was experiencing delays, I didn't find the length of time spent in the review process at all unreasonable—particularly if the journal submission process allows potential reviewers to see the manuscript before electing to accept or decline the invitation to review.

The poster believed that it the journal should have sent the rejection much earlier—but I figured it might take six weeks or more in some cases to find reviewers (particularly given that the paper was somewhat outside the "core" themes of the journal).

So what is a reasonable amount of time for a journal to spend before returning the paper because of the inability to find peer reviewers?
If anything, my reaction is that two months is too soon to absolutely give up on trying to find an appropriate referee.  Generally (at least in my discipline) people are given a week or two to accept or decline a referee request, so two months probably means that not more than ten or so people were asked.

My understanding of what it means to be an editor is that they should in principle be working harder to find a suitable referee.  However, if the editor is having an especially bad time of it, it may be worth letting the author know: maybe they will choose to resubmit to a different journal or to switch to a different editor at the same journal.  The latter happened to me years ago: the editor that I submitted to asked one person to referee, got referred to a second, and so forth.  He reported failure to me after the loop closed up.  But the failure was conditional failure, and he gave me the option of switching to a different editor at the same journal, which I did.  (Unfortunately after that the paper still was not treated well, but that's not part of the present story.)

Academia is specialized, but it's not that specialized: probably most journal submissions have at least 100 suitable referees in the world, right?  If so, then two months is way too soon to say definitively that a suitable referee could not be found.  In my experience, editors often "sugarcoat" the things that they tell authors -- probably because they didn't like the hate mail they got from their earlier, more forthright approach -- to the extent that if you take literally the reasons given for objection, they often sound rather silly.  In this case I might suspect that "I spent two months and I couldn't find a suitable referee" is meant to be code for "I asked enough people whose opinions I trusted, and while they were not willing to formally referee the paper, they told me that it did not interest them and thus probably did not merit publication."  If the editor is himself making a judgment call about the suitability of the submission, in my opinion that's fine but they should be upfront about it.  

Unfortunately the current culture of academic journals makes extracting "the real story" from the editor almost prohibitively difficult.  In practice it is probably in your poster's best interest just to try again at a different journal.