I would say, keep the time duration of 2 months.
For first 2 months,
1. Keep posting yourself and with some aliases, tell your friends to keep posting quality and related stuff.
2. Start advertising your forums and keep some good contest where the winner will be adjudged by the no. of posts.
3. Write a good article or newsbyte announcing your forums. Highlight the advantanges, uniqueness and information richness of your forums.
4. Keep updating always. Provide some unique feature in your forums.
5. Keep avenue, arcade section in your forums. People tend to hang around a bit longer with this kinda stuff.
6. Exchange some quality links and stuff, this should bring in nice bit of traffic which will be converted along the period of time. More informative and unique the forum, more will be member conversion rate.
I suppose this should get to good amount of members for your forums. Keep a target of 750+ members by 2 months end with more than half of them active and posting. I know I have followed this system and thus far has worked.
Hope this helps ya out.
Have a nice day.
Regards.
Admans.