It emerges as a need
Hello there,
I've been really "doing" GTD for about 1 year now, and where it has taken me so far, is into a greater and greater need for a clarity at the higher levels.
When I first began to use the GTD tools, I had a tremendous "A-HA!" experience seeing and experiencing for the first time the difference between a true next action, and a project in disguise - as somone once referred to their to their todo list as "an amorphour mass of un-doability".
After a few months, I broke through another glass ceiling, as I started to realize that I had effectively shifted this amorphous mass up to a higher horizon level - I had to go back and do for my 10K / 20K / 30K levels, what I had originally done for the runway / 10K level : I had a whole mess of 20K-and-up material crammed into my project list, which was making it impossible to ever complete a weekly review in a reasonable amount of time, and making it a very frustrating experience, as it still felt like my life was a blob of over-whelming un-doability.
At that point, I seriously tightened up my 10K level (project list), by ensuring that every project on the list had a very concrete, clearly articulated end-point, that will occur within the next few months. Anything that more realistically was approaching the 1 year time frame got moved up to the 30K level. Same for all the roles, responsibilties, and relationship stuff which was populating the project list - I have redined it appropriately onto the 20K list, and ensure that there are active, concrete projects at the 10K level moving each one forward. (I also got a better grip on using the someday/maybe list in a more active and useful way at this point).
As the weekly review is best done aproximately every 7 days, I am still exploring what time frame is best for reviewing these higher level horizons. I suspect it will fall somewhere between monthly and quarterly, but I will see after I have more experience working from these horizons of focus.
As David Allen talks about the inherent "scuzz factor" we experience when our internal standards aren't being met (such as brushing teeth daily), I have found that my scuzz factor has naturally shifted up to these higher levels of focus, and I am not following an external formula of when I 'should' do these reviews, but the internal need to be clean at those levels, as well as ensuring an alignment between all the levels.
Jeff