I like to run a 7 day pre launch, and 7 day launch period.
I've found that pre launch periods longer than 7 - 10 days have a tendency to peter out, prematurely, but that's just my opinion.
I'm not saying that you can't allude to the project's arrival to start the buzz process long before that, I'm talking about partner involvement.
That's assuming that everything is in place prior to the 7 day period ... campaign management software is in place, JV partners on board, comps delivered, promo tools ready to go, at least a rough of the final sales page is available, etc.
I also find holding a 7 day affiliate contest to run, commensurately, with the 7 day launch period is usually very effective.