It doesn’t really take all that long to build a forum, unless you’re using IPB then it could take an indefinite amount of time. Assuming your’e using good software like Xenforo, the building stage should be relatively short, a few hours to a day or two depending on how customized you want to skin it.
Sometimes I get bogged down in the naming process. Coming up with a good name for a site isn’t easy these days, as it’s only about ten bucks a year for domainers to sit on good names that aren’t being used. On a few occasions I’ve spent hundreds to over a thousand dollars to a domainer to “buy” a domain they were holding. You don’t always need to do that, but if you want a good name it’s often the only way.
Coming up with the concept for a forum should be simple, but from my experiences reading what people write about here at TAZ, I realize that it is not simple for all people. In a nutshell, what you should be thinking with your concept is that you want to start a forum where the audience is big enough to have reaching critical mass be a possibility, without going after an audience that is so large that getting it to critical mass is going to be like bailing the Pacific Ocean with a thimble.
Building the community is where you really need to put your time and focus. Almost every forum I’ve seen that has failed, you can obviously point to the lack of time put into community building by the owner. You can not absolutely just build a site and expect people to come join. In the beginning, you need to invite each and every new member personally to join and ask them to post. In other words, take (almost) all of that time you were going to put into planning your forum, building it, etc, and put that time into community building. Yes, get a good name, yes install the software, yes get a decent logo and customize the template somewhat, but don’t dwell there, dwell on the community building. Otherwise, what you end up with is a pretty site with nobody using it.