i dont think there is a time set, i got one of my sites listed in a week of submitting it, i think it is very benefical as google and a number of search engines crawl DMOZ first and all dmoz listings, google used to crawl my site a few times a month, now it crawls it over 40 times a month. also my site is now crawled by loads of other search engines, also alot of website directories use dmoz results, so you will have more links back to your site, submit once and wait a few months, if you keep submitting it you will be sent to the back of the que everytime you submit. if its been over 3/4 months you can ask on the dmoz forum
just to show you the difference it made to my site:
before dmoz;
2 different robots* Hits
WISENutbot (Looksmart) 3+2
Googlebot (Google) 3+1
and last months after being in dmoz;
10 different robots* Hits
Googlebot (Google) 874+56
Inktomi Slurp 86+52
Unknown robot (identified by hit on 'robots.txt') 0+107
Unknown robot (identified by 'crawl') 24+5
Alexa (IA Archiver) 0+24
Netcraft Web Server Survey 12
Jeeves 10
Unknown robot (identified by 'spider') 4+4
WISENutbot (Looksmart) 2+4
Unknown robot (identified by 'robot') 2+3