The whole idea of GTD context lists is to have a list of like-sized tasks that are actionable in your current context, so a well-implemented GTD system should enable you to fill these moments with productive work.

Let's break that first sentence down to yield some more practical advice:

like-sized tasks

There's no right answer for how long these tasks should be, but 10 - 20 minutes seems like a reasonable target given that duration is a common window of time you're trying to fill. This also jives with a pomodoro-technique workflow, which advocates bursts around 25 minutes. (In practice this time constraint serves as an upper bound, not a lower one. Lots of tasks take < 25 minutes.)

The reason for this advice is two-fold. First, the more tasks that fit inside your available time span, the less searching through your context list you'll have to do to find something to do. Second, if a context list is allowed to have massive actions on it they become too daunting and never get started. A big action is probably just a project in disguise, and should probably be converted into an action to properly define the project.

actionable in your current context

This one is all about not seeing things on your list that you're not in the right place to do. I don't want to review all the house chores while I'm sitting at my desk at work. Not only is that more stuff to look through but generates mental anguish from knowing the things I want to do that I can't.

The last important tip is that sometimes you're not going to want to do something on your list. That's a perfectly good time to clean a drawer or catch up on your RSS feeds.

These are my opinion,

(1)Brief

I suggest you if you have a brief time in your day, get some fresh air or do some training. Or play some brain games. Whatever you’ve got coming up in your work or personal life, it can benefit from a brainstorm. And that doesn’t take long.

(2)Randomly dispersed

If you had a time like this, that means you couldn't schedule good your task list. But this can be happened. Don't worry. So make a task list for your free time. And keep it. Sometimes free time is boring.

(3) Stuck between other tasks

Reschedule, reschedule, reschedule your task list.

But the important thing is;

If you’ve got a big of free time, the best way to put that to use is to relax, have fun, decompress from a stressful day, or spend time with a loved one.