I had one batch that was still flat after about 4 weeks in the bottle. I decided it just wasn't going to happen. I let it sit for a few months and WOW, it was the best beer I ever made. It just took a long time.
In the meantime, start another batch so if the first one takes a few months, maybe the second won't and you can be drinking homebrew.
And speaking of old homebrew, I found 2 bottles of the 2nd incarnation of that same recipe in my basement after thinking they were long gone over a year. Still the best. And I found some old nut brown ale just the other day and I put one in the fridge. Maybe I'll try it right now!