It's part of the weekly review, except not every week. Monthly items would be every four weeks or so, and so forth. Also, at these levels, it's more of a gut feeling than a set interval.
To integrate it into your system, you could add a higher-level review folder into your Tickler file. Have your latest ideas about each level inside, and tickle it a month out. When it pops up automatically, quickly run through your current state for each, and see if you need to update any level. This gets it in front of your face automatically, and then you can choose to change something, or simply review/remind and then plan it forward another month.
How does one integrate the 'Levels Of Work - The Priority Hierachy' (50,000, 40,000,..........) into ones GTD system. How often is this being reviewed ? How is this stored ?
I manage the Runway in Outlook and a Palm Treo. I have about 95 Projects right now and the NA's associated with them (plus the one step Next Actions).
I keep the 20-50,000 ft. Focus items in the Notes section of Outlook. I like to review the 50,000 ft. Horizon during my the weekly Review but deal with the 20-40,000 ft. views monthly, or, when I'm being nagged internally by something on them or that I think should be on them.
I am working on taking time to drill a little deeper into the higher levels so that they finally make it to my Project and NA lists and are moved forward on a consistent basis. I too often fight the fires of my days rather than think through those things that will lead me to the places I want to dwell contentedly.
GTD has helped me tremendously on the Runway, and I live there most of the time. But when I do jump up to those higher levels, I always come away convinced that there is more, much more, to life than just cranking widgets.
I posted all of my higher-level goals on pieces of paper on my studio wall. I look at them during my Weekly Review, certainly, but I also glance at them throughout the week. This reminds me of my destinations.
Those higher levels are basically checklists that are reviewed whenever you see fit - monthly, quarterly, etc. So that's the tool and behavior. That said, I think Allen's method is strongest (very strong, actually) at the actions and projects levels. I think other systems have a cleaner connection between those two and goals, for example Sally McGhee's work. But for me, just managing actions and projects has made a tremendous improvement in my life.