Getting a subforum for a group game (i.e., the process of going from a small group game to a large group game) is mostly taken on a game by game basis, but we generally look at the ratio of IC to OOC posts when reviewing a game's activity. Games with a very active OOC thread would require significantly more IC posts than one with a less active OOC thread.
For example, if a group game is composed of 500 IC posts and 800 OOC posts, we would probably ask the GM(s) to make the request again when the number of IC posts is closer to 1000. A game that has 750 IC posts to 250 OOC posts would be more likely to garner a subforum; but again, it's really taken on a game by game basis.
However, we generally like to see the number of in-game posts to be very close to 1000 when being asked for a large group subforum.