I think it depends on how much time and money you have to spend on promoting your forum. With a large budget, I'm sure you could get a forum going in a week.

On average, I find that it usually takes a year before a forum can live on its own.

I was a part of a forum (which is now closed), which was a Graphics Forum. People could request 2D graphics to be made, some others could fulfill them. There was no monetary gains involved.

There were no PPC advertising, because this forum was advertised in a game creation community, where people did need 2D sprites or 3D graphics. Also, people willing to help them were present too. Within a few days, the milestone of 200 was crossed, and so was the first 1000 posts.

But things started dying out, as there was nothing new which could keep them involved. Too many other similar forums entered the scene, and people were essentially scattered. Moreover, the original game creation community had a Graphics sub-forum, and most of the people didn't find a need to join ours.