It never is productive. I usually already know something or have no interest in knowing it. If I look at something boring for too long, I end up on some deep jungle of thought on something completely different.
What I do is put in a good 10-25 minutes looking at what I'm studying, then take an hour or two break playing and doing other things.
Being under a time crunch helps with motivation.
I know people who can sit down, study for hours, and when they're done they've memorized everything. I've memorized whole monologues for plays and musicals, but I'm still impressed. I wouldn't even be able to go through all the material without sleeping.