No, it should not take that long, but my experience (hosted here since 1998) has been that getting mailboxes "live" is the thing that takes the longest with Dreamhost.
I have been fortunate enough to have a new domain propated, users set up, and all else in order in under an hour (yep!) on numeroius occasions with Dreamhost, but have had to wait for in excess of 24 hours for mail to be usable on some occasions.
I have no idea why this is; it doesn't seem like there is any reason for it to take so long. It almost makes me suspect that this is something that, for some unknown reason, might require a "human" involvement at Dreamhost, though I can't imagine why that should be the case.
Obviously, YMMV, but I have had it take as little as less than an hour, to over 24 hours...and I can't annecdotally account for why sometimes it is so much quicker than others (though I don't see why a mailbox shouldn't be "all set up and ready to go" by the time DNS propagates!)