How come it takes so long? How in depth is this review?
Fair question.
The review itself can take a mater of minutes to an hour or longer. Factors include the complexity of the site, the nature of the site (is it in an industry that is rife with affiliates and spammers), the correctness and completeness of the suggestion.
If someone suggests, say, a restaurant site that has 4 pages, it is suggested to the correct subcategory within the correct locality, and it has no broken links, no nasty content or redirects, it is not a mirror. Just a nice clean website, the review is fairly fast.
If the owner of this restaurant website decides to ignore our guidelines and instead of sugesting it to the locality where it physically resides, and suggests it to the state level, they have induced additional time in teh process because an editor needs to find the site, move it to the correct locality and then it still needs to get reviewed. I have spent many a multi-hour editing session doing nothing but moving mis-suggested sites.
The real holdup in reviews is simply one of available resources. There are around 7,000 active editors, none are what you would call full-time, none are paid to edit, it is done as a hobby or as a labor of love (for some of us it borders on insanity). Heck, it can be weeks or even months between logins for some editors.
There is probably an 80-20 type rule that applies -- I'd bet that 5% of the active editors do 90% of the edits in any given month. That's just speculation on my part.
Also, there is much more to editing than just adding sites. We clean out spam, design categories, hunt for sites that were not suggested, hunt for bad links and sites with changed content.
Bottom line, there is a lot of activity, but not all of it is focused on adding sites that were suggested. Prior to the great crash, we had sites awaiting review for more than 3 years.