In terms of how long it takes to develop a character, to some extent, it is what Sakurai and co. make of it. You could argue that all the characters they're patching each update still aren't "finished," because there are still things you can do to create a more refined playing experience with those characters.
At the same time, between whenever Mewtwo started development and his release in April, the team had time to get mostly, if not entirely done with four characters, at least to a point where they could be released. It's been six months since then, roughly a similar time frame, and unless they release four more characters all at once next Direct, they'll have even more time if they plan for more than the two Mario spots.
I personally doubt they've been twiddling their thumbs since June with characters, and that a few measly ported stages and costumes took up that much of their time. If it's such an arduous task to "keep the team together," why not make the most of that time by being productive with at least one or two new characters?