Depends what an editor feels like working on. I have unreviewed ranging from a day or two ago, to much, much older. I edit many different categories - so if I feel like working on category A today, and all it has is a new site from yesterday - it will probably be reviewed. And the sites in category B, that might have been waiting for months have to wait for another day.
Also, some sites are easier to review than others. Some sites clearly belong in the category, don't hint of being spam, have no bad history associated with them. Others may be unclear in what category they really belong in, might have had a history of being up and down, might be currently not working, and get postponed.
In some cases, no proper categroy exists, and requires an internal discussion to create a new category.
Somtimes certain categories just don't have any reviews happening, since no-one is really interested in reviewing sites in that area. We don't force editors to work anywhere or in any order.
