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?