I thought about opening a forum about a month or so before actually starting one. I really enjoyed that time, going over various ideas in my head as to how I wanted things. I didn't bother rushing things. It seems like a really long time now to spend on just the planning stage for a new forum, but it was a good thing because it gave me a chance to think things through rather than rush in.
I normally think about it for a few days. I like to take things quickly because the time I spend thinking about the idea could be used to get members into the forum. However of course its a good idea to think about it thoroughly before starting a project as otherwise, it could go horribly wrong.
I sit and contemplate on making one and than I never do it seeing as most of the forums I did were failure. It just wouldn't be smart for me to try at another one if all the other's were big failures.
I would at least spend 2-3 days thinking of ideas for the theme, advertising once the forum is ready, names for sub-forums, user ranks, writing up the rules, and all that good stuff. I think that's reasonable. It's not good to rush things because issues can arise.
It took me about 6 months before I thought about launching Webmaster Lounge. Had a lot of planning to do and research out sites/forums within the same niche.
I've been thinking about the forum I'm building for 2-3 years now. It's gone through multiple name changes, software changes, ideas, and potential staff teams. Simply because I don't want to put something out there that i'm not 100% proud of, and I took some time off from foruming as well.