ASAP. The sooner you start the sooner you get posts in there and get the ball rolling. Starting a forum is always rough and the only way to make it look good is to get people in there and to get them to post. Whether you start with 200 uniques a day or 2,000 you still start the forums off with zero posts. I have a forum which gets 400 uniques a day and gets an average of 100 posts a day and I see no reason why you shouldn't be able achieve similar results.