I once knew a guy who spent "many hours" a day promoting a general chat forum. I think it took him a month or less, but he has a large following online by having tons of online friends, and is very active on "many" forums and did a lot of post exchanges and advertising on other forums to make a forum successful.
It all depends on your genre. A forum in a unique genre with low competition will do much better, much faster. A forum in a more "general" genre will find it needs much more work to be successful.
The most important thing you can do is to continue to add unique discussion topics to your community to make your guests want to join up and respond.