Assuming Smash 4 and 5 don't get delayed, it could come out anywhere from (my thoughts btw, so take it with a grain of salt) May 2014 - December 2014. If that is the case we have anywhere from 9 months to 17 updates.
I'm going to do some foolish calculations here. Smash 4 started development soon after Kid Icarus: Uprising was released, which was March 2012 correct? That means that after 16 months or so, we have information on roughly thirteen characters. If we went on a monthly basis, we would have 17 more characters revealed, giving us a total of 30 characters revealed on the site, leaving anywhere from 10 to 20 as unlockables.
For Brawl's site, we were given 25 character reveals (or 27 overall) before the game released, with 12 characters as unlockables. With this as a comparison, more than 15 would be a LOT of unlockable characters. I like to unlock things, but there would be a lot to do.
I predict that we will see more than one per month, or some months with several, some with maybe none. I think we'll see more characters revealed than the amount of roster spots in Brawl (35), as the Brawl did with Melee (which had 25 roster spots).