From a site owner perspective, that's good advice.
Look at the other sites in a category, and if you can put some information on your site that none of them have, that gives your site unique content, which is what we look for.
One of the categories I edit in is about water gardens. Because I have one of my own, I'm very interested in that particular subject, so I'm very aware of what type of information is going to be useful to me as a web surfer looking for specific information, and what information is just generic information that every website on that subject contains.
If 20 other sites have the same information, why would I want to add another one? To be nice to the website owner? I'm not interested in being nice, I'm interested in finding new, unique information for the web surfer, that can't be found on the sites that are currently already listed.
I already know a lot about Water Gardens/ Koi Ponds, so when I find a site on the subject with new, interesting information, or a better way of presenting it, I get excited, it's like finding gold. It has unique information that is of value to web surfers.
If I get a site suggestion to that category that has the same old information that every site has, and it's just loaded with advertisement links to other sites, and presents no useful, unique information from the site owners personal experience with the subject, I'm going to delete the site suggestion.
It adds no value to the category, and no site has a right to be listed.
Back to how long it takes to get listed. I may find myself getting very busy editing in another part of the Directory, and as I can't be in more than one place at a time, I may not get around to editing in the Water Garden category for several months.
However, no editor owns a category, there are 200-300 other editors who can take a sudden interest in the Water Garden category and pop right in to do a little editing there, while I am occupied elsewhere, and they are most welcome to do so.
To quote editor spectregunner:
There is no magic mathematical formula you can apply that will let you calculate when the next editor will visit a given category (700,000+ categories, ~7,000 editors) nor what a given editor will do when and if they visit a given category. Try to guess when the next time a given hummingbird will visit a particular flower on a particular bush. Calculate with or without the neighborhood cat making periodic visits.
Just to add to that, those 7,000 + editors all have varying levels of editing privleges, some edit in one small category, and other more experienced editors edit in thousands of categories.