Dallas:
I'm at about 8 - 9 weeks, and I have so much stuff cluttering my world that my original 3-6 month expectation has been pushed farther and farther out. Naturally the implementation process depends to some extent on how many years one has been doing things in other ways. For me, at 56, I have a LOT of things to undo and in some cases open loops going back longer than I'm even willing to admit to myself right now.
There have been moment of frustration (almost depair) as I realized how some areas of work and life have gotten so far out of alignment, especially since the application of some basic GTD concepts on the front end would have made such a huge difference. I have actually experienced moments of frustration that made me almost ask "Is it worth it to make changes this drastic?', until I come back to the realization that it's either get on with the task or "go live on the beach & make hats for the tourists" as David says in the FAST CD.
At the same time, I'm seeing immediate benefits with my tickler file, 2-minute rule, empty email & voicemail in-boxes, more efficient use of my Palm, organized context lists, etc. This makes me want to implement GTD even more - I can sense what it feels like to gain control over some areas I had previously written off as hopelessly disorganized. To know that degree of organization & control is possible across the board is exciting, but there's a price in time and resources. In any event, I'm much more relaxed and focused and can honestly say that there are things which I will never let crawl back up in my head and distract me. Whenver they try distract my mind, I just remind them that they're on their proper list, they will come back around at the appropriate time, and they can just leave me alone right now.
I'm hoping to beat two years, but at this point I would be surprised if I can be where I want to be in less than 12 months, (maybe even 18 months).