I think i read on the DMOZ forums is was 3 months. IMHO i would never submit every 28 days. Might label you spam or put you back at the end of the cue.
I think these guys over at DMOZ feel like gods. I wouldn't put it past them to label you spam if you submit too often. The DMOZ guidelines say you can submit every month but editors say: "That practice has never been acceptable to us and could well have been counter productive" <---taken from the DMOZ forums. Their Guidlines State How long should I wait before I resubmit? The simple answer is "forever". The ODP submission guidelines used to say to do it every month, but this has been rendered unnecessary by changes in how the ODP works. Once a suggestion has been received, it will never go away. It will sit in that category, awaiting review. Suggestions do not expire. So the preferred method is now to suggest the site to the category and wait at least a month. If, after a month, the site has not appeared, then feel free to submit one more time only. Please note that a new suggestion to the same category overwrites the previous one. Editors have many ways of sorting the pool of unreviewed sites in a category and one of them is by suggestion date. If this is the case, then a re-suggested site will show up further down the list and may end up taking longer to get reviewed. - - - -- - -- - -- -- - -- - Now we all know, including me ;-)