Always timebox your work. However the first question should be is this worth your time, and the immediate follow-up question should be how much time would this activity consume. Timeboxing is the ultimate answer to our limited time. If you make it a practice to ask these two questions for every activity you do, you'll achieve more and start to experience a feeling of accomplishment even with the smallest of things. Small things can help you complete bigger goals.
For example, I timeboxed to complete this post to 20 minutes, which includes writing, editing, selecting an image (which is such a waste of time, due to the current theme limitations. I'm exploring text-only blog themes and hope to find one at the earliest. Until then I'll reuse a repository of images) and posting it on my blog. For me, writing often matters and this perspective addresses the other question, of whether it's important in the first place.
First pick the right problems, the right activities to do, and then focus on those problems intensely. That’s all there is to work. While timeboxing is one thing. What to timebox is of paramount importance. The answer to this is in the fundamental stoic question, posed by Marcus Aurelius. "At every moment, ask yourself—Is this necessary?"