Without being rude, the answer is somewhere between two minutes and twenty months. There is no average, only a range. Sometimes an editors will work furiously in a category for a week or so, tire of the content and go off to edit elsewhere and not come back for quite a while.
I have, as recently as two weeks ago, dropped into a category and found site suggestions that were more than a year old. And this is within Regional/ where we generally are able to get to sites faster than in other areas of the directory.
I know that four months seems like a long time to you, but in "ODP time" it is not very long at all. Also, wait times have little to do with the size of the category or even the size of the heap. Wait times are (usually) driven by four factors:
# of submissions and amount of spam in a given category
# of editors
Editor interest (in the category)
Other things on the editor plates.
For example, when I logged on a few minutes ago, I had nearly 1,100 site suggestions vying for my attention, plus an entire heap of sites that were miscategorized several years ago, plus some sites that are reporting errors, plus, plus, plus....So I go where I think I can do the most good on a give day or editing session.
I suspect that it is similar for most other editors.
This is one of the reasons why, if you peruse the threads, you see us often as people to apply to become editors. If someone applies and takes the smallest category way out on a branch of the tree, and they do good work with it, it lessens the amount of work for every other editor -- and helps us, hopefully, have time to work on some submissions.
A long answer to a short question, but I wanted to give you a feel for some of the behind the scenes dynamics that can impact the amount of time a site waits for review by an editor. I hope you find this helpful.