It depends on the type of forum, and the content at hand. With my largest forum I opened up the same day I made it, it was a 4 hour project and I mass promoted it and gained around 1k members the first day which started posting like crazy. I try to feel for my audience first, if I know something will be harder to promote I will give it time and build up content, if I feel it'll be a success I'll start it same day.
This site I waited until my custom plugin was finished developing. I'm not too worried about content as of yet, however I do want and strive to release at least 2-4 premium articles a month. Other than that I'm not worried as I'm focusing all of my time on the SEO to gain members over time.