Heck, when I started, I put stuff up as I went, and it definitely turned out great. People were posting and talking about the site before we even had an actual domain.
Personally it really depends on the person. Some people can get a stunning website done in about a month. Some can get it done in about 2-3 hours.
For a forum, there is not much that you have to do other then create the custom theme and install mods and set everything up. So I would have to say that I would spend about 2-3 weeks creating a forum.
I know one thing a person should do when launching a forum at all is put some content into it, and more than one post per forum, preferably. I used to be bad at that, but now days it aggravates me more than anything when I stumble upon a seemingly good forum, only to find one measly post in all categories, except perhaps Intros and announcements. It really throws me off and I honestly think that a person should spend some time getting content on their site before launching If you're that hard up for ideas, do a few post exchanges to get some form of conversation going.