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Take for instance this one site I submitted, it took 4 months. Finally it was added to dmoz. After some research I had someone check the category logs and to my surprise it wasn't that my site wasnt good enough, or didnt make it in, it was just that the category hadnt been edited in a while.
It takes some time, even e-mailing the editors a few categories up will take some time because there are a few other categories with many requests still pending. It's all about time. If you dont have the time you can send notes to the editors, but if you do it - be sure to do it nicely and ensure you dont include anything like 'WHY ISNT MY SITE ADDED IVE WAITED 4 MONTHS!' because they dont have to, and they don't get paid for it ;)
I recieved a reply within half an hour and would definitely reccommend it as a first stop. There a number of editors available there so it can be faster than emailing one specific editor. Resubmitting can put you to the back of the queue again (apparently) so it's a good idea to check the status of your application first.
Shelley