30,000 units is a LOT.
If you look at one of the biggest, most "mainstream" board game makers right now, Cool Mini or Not, their games get around 20,000 backers on Kickstarter then go to retail where they sell some amount of copies.
Basically 30,000 copies of MvM is equivalent to what a company who is expecting their game to be REALLY popular would order. I could easily see the first wave selling out and probably the second considering the amount of LoL players who will buy it just because it's connected to the game.
Also a factor to consider is how much value there seems to be in the box. It's only $75 and it contains quite a few miniatures, I wouldn't be surprised if subsequent runs cost more.
As for how long it would take to get more units in stock, it's about a month to ship freight from China to the US by boat. Various other hurdles like customs can add anywhere from a few weeks to two months so it all depends on when Riot orders it. If they order more as soon as the second wave sells out in December you're probably looking at March before you could get a copy in your hands. Be aware however that any subsequent print run isn't going to be anywhere near the size of the first so if demand is still fairly high you might have trouble getting one then as well.
While Riot probably doesn't intend to make this game a limited edition sort of thing there will come a point where there is still demand but not enough demand to make it cost effective to keep making the game. When that happens is anyone's guess but if one wanted to speculate on a game, this might not be a bad one to look at long term.