I have got an "Getting Things Done" ebook, so I can't provide the exact page. The quoted text can be found in Part II, Chapter 7, section Someday / Maybe List.
I think the section you're talking about is the section I'm finding which reads:
“Reassess Your Current Projects Now’s a good time to review your Projects list from a more elevated perspective (that is, the standpoint of your job, goals, and personal commitments) and consider whether you might transfer some of your current commitments to Someday/Maybe. If on reflection you realize that an optional project doesn’t have a chance of getting your attention for the next few months or more, move it to this list.”
To me, again, a key there is "transfer some of your current commitments" and "an optional project". One is re-negotiating a commitment by deciding that it's not required, and figuring that it can always be picked up in the future. It's about the commitment level to the project, not just about where the project is tracked.
So if "Learn Linux" is a fun side project, but there's no time to do it now, re-negotiate the commitment with yourself and kick it to "Someday / Maybe". But if learning Linux is going to be required for a job function in the future, making it a *required business project* (i.e. "I can't re-negotiate this one"), moving it to "Someday / Maybe" doesn't seem to comport with the system.
but if you put on the list the project like "Learn Linux / Polish cuisine / Sing like Paul Anka" or "Write a book", then: or realization of all of these projects will take more time - longer than a year or most of them will not be carried out during the year at all.
I agree that larger things will take more time - but what I seem to be hearing / reading over the breadth of the GTD material is that the choice is one of active commitment vs. "it would be nice if ... ", and managing the amount of things you're actively committed to if the time available doesn't comport with reality. From the intro:
"It's possible for a person to have an overwhelming number of things to do and still function productively with a clear head and a positive sense of relaxed control. That’s a great way to live and work, at elevated levels of effectiveness and efficiency. It’s also the best way to be fully present with whatever you’re doing, appropriately engaged in the moment. It’s when time disappears, and your attention is completely at your command. What you’re doing is exactly what you ought to be doing, given the whole spectrum of your commitments and interests. ”
So if you decided (firm commitment) that you want to learn Linux, learn to cook Polish cuisine, and learn to sing like Paul Anka, can you do all of those to any reasonable level of completeness in six months? Probably not. But if you have "find a Polish cooking class", "research a top-rated book on Linux", and "research singing lessons" on your "@ Computer" list, you might find yourself in a foggy mental state some Friday night when you can't do much else regarding your job, and decide to do those things. Or if you're walking by a book stand at the airport you might see a "Linux for Dummies" book that you (a) recognize is relevant to one of your current commitments because you've been reviewing that commitment weekly, and (b) decide would be a good first step.
Even if you only move forward an average of 20-30 minutes per week on any of those items, you could move all of them forward a little bit each week.
And if you *can't* move it forward because the time just doesn't exist, my understanding of the GTD mindset is that you *either* decide that you're still committed to the outcome (and thus either keep it on your list or do the thinking to determine *when* you might want to pick up the thread again and "tickle" it into the future), or re-negotiate the commitment and move it to "Someday / Maybe" as an item you might want to consider at some point.
Continually scanning your "Someday / Maybe" list for required projects doesn't make sense to me based on what I've heard / read from DA.
Your mileage may vary.