Well, arguably there are several averages. The median is something like five years, or infinity (depending on whether the rejected submittals are slightly less than half of all submittals, or slightly more than half, and ... I'd bet small sums on the latter, I think. The geometric and arithmetic means are both infinity, and will be so long as some sites are rejected. Oh, you can count up submittals and divide by number of sites reviewed per day, but ... who knows what the distribution would be?
My best guess at a way of estimating the amount of time remaining until a REVIEW (not, of course, a listing) is to the amount of time the submittal has ALREADY been waiting, and multiply it by some fraction between .5 and 1. This, on the face of it, is absurd -- but it incorporates what is, I think, the most significant aspect of any theoretical estimate.
My suspicion is that people think the longer a submittal has waited, the closer it is to being reviewed. Whether or not people actually think that, I firmly believe that it is exactly the opposite of the truth. The longer a site has waited, the LOWER chance it has of being reviewed, and therefore, the longer one should expect it to wait for review.
But for all PRACTICAL purposes, infinity is the right number to use. That's right, infinity. Assume the site will NEVER be listed in the ODP, and then take steps based on that assumption. Regardless of what ACTUALLY happens, that will turn out to have been the best choice.
