Replying to myself: Let's imagine that you're working toward becoming a concert pianist.
1) would be "From 12:15pm to 1:12pm I will practice Brahms Third Racket."
2) would be "I practice every Monday, Wednesday, and Friday from 1pm to 6pm."
Edited to add: I realize I left out the Pomodoro interpretation. Even though this may be the most accurate definition of time-boxing, it's not what I think of as time-boxing, so I left it out in the cold.
It certainly is a battle between “none of my actions have to be done right now, so I’ll wait” and “I don’t have unlimited time, so I’ll schedule my actions.” I lean toward the former, being lazy and entirely incapable of acting in the face of even the slightest unreal deadline or schedule. Anyone more disciplined than I am (hopefully most people!) may very well use time boxing without problems.
For things like instrumental practice, I find that next actions lists are just as good as, if not better than, time boxing. Think about it: The end result—you having played guitar for 2 hours, for example—can still occur regardless of whether you schedule time for it on a calendar or put it on a next actions list. The benefit to using a next actions list for organizing such things (day-specific slots on a calendar work too, if they're deserved) is that you can stop whenever you need or want to and not feel guilty about having broken a false time-specific commitment.
Now, if playing guitar for two hours daily is critical to some urgent project and/or higher outcome, and you’re so busy with absolutely every other available time slot that you must block time for it on your calendar, I can see this as being proportional to the need. Such an instance would not represent a false time-specific commitment, but a very real one—hence why I don't call these instances time boxing.
If, on the other hand, you have enough time in your typical day for you to choose between several different opportunities for you to play guitar for 2 hours, yet you’ve blocked out specifically 5–7 p.m. for the task, then your map no longer matches the territory, and you've violated the have-to nature of the calendar. I would prefer day-specific slots or actions lists for such cases, depending on the nature of the commitment.
Cal Newport missed this in his article related to the subject,“Getting (Unremarkable) Things Done: The Problem With David Allen’s Universalism”. He falls victim to believing that next actions are exclusively brief, mindless activites like "Refill stapler," "Water plants," or "Email John re: budget" (it sucks, too, because this fallacy seems to drive away from GTD many people who would've otherwise benefited from it). In truth, there is no inherent time limit on next actions; they're simply the next physical things you must do to move current reality toward the one you desire. They might last 5 seconds, or 5 hours. They could be "Draft next chapter in book," "Watch Star Wars: The Phantom Menace," or "Mix song X" (assuming those are the next actions needing to be taken on projects and not projects themselves). The only time limit that may exist for them will be born by other commitments, which, in dire straits, can warrant time-specific slots.
To be super clear, my definition of time-boxing is the blocking out of a time-specific slot on a calendar for a commitment that is not truly specific to that time. I find that to be extremely unproductive in my own life and counter to a lot of what GTD is about.