Hmm, very meta question.
Setting up GTD, as in reading the book, setting up the boxes for your actions, and going through the first setup process, would be a project. Once you are set up, you tick the box.
Maintaining that system and developing habits is more a 20.000 if not higher thing. Being organised, with whatever system, is definitely an area in life one needs to keep on top of. Getting black belt at GTD is usually a very long term goal, think a few years to get really good at it.
Since I got into "getting more done / getting more to be done" cycle using other methods, putting GTD on was, for me, a matter of survival... and I guess I have my Pareto's "80% result" with the "20% effort" I spent, so... I may not need all those years!