I just went through the process myself. It took six days from the time I submitted to iTunes to the time my video podcast (iTunes link -- finally) appeared on the iTunes Music Store. During that time, I wasn't sure if there was some problem with my RSS feed, whether I'd used the right tags, or whether the delay was just part of the iTMS submission process. I finally got a form email from Apple telling me that my submission had been accepted, but it took another day after that for the podcast to show up in an iTunes search.
If you're going through the iTunes submission process, the iTunes tech specs and the Apple Discussions Forum for podcast producers may help you figure out the process. But be prepared to wait.
8/23/06 Update: After reviewing my records, it looks like there were fewer days than I'd originally figured between my submission to iTunes and the day I got my approval email. It came down to one business day because there was a weekend in between, with another day before the podcast came up in an iTunes search. My apologies to the good folks at iTunes. More news since I wrote this post -- my stats show that the majority of my podcast's hits have come through iTunes. So I'd say it's definitely worth submitting your podcasts to iTMS.