I break assignments, and engagements, into manageable pieces, for a number of reasons. First, 30-45 minutes each day is better than 3 hours in one day. Why? Your brain drifts after a while, so although you are getting practice with the latter method, you aren't getting focused practice. Intensity of effort, not volume of effort, is what gets results. Focused practice is what helps you break things into components, spot subtle differences, discern patterns, and sharpen your attention to detail. It helps you form new habits.
Building new habits also requires persistence. The key isn't how much time you put into test prep on a given day, because eventually you run into the law of diminishing returns. Instead, habit-forming comes from repetition - specifically, timed repetition. Studies on human long-term memory have revealed that timed memory techniques (which focus on "ringing the bell" just before something is ready to fade from memory) are more effective than volume repetition. Meaning, you are better off "ringing the bell" once a day for six days than six times in one day. Shorter, daily deadlines also make procrastination less likely, increasing the odds that the student will devote the necessary time to build the new skill:
“Even if you know what's critical, without deadlines that create focus, the minor tasks forced upon you (or invented, in the case of the entrepreneur) will swell to consume time until another bit of minutiae jumps in to replace it, leaving you at the end of the day with nothing accomplished."
- Tim Ferriss, "The Four-Hour Work Week"
So what is the takeaway? If you want to increase productivity, you can either limit your tasks to only the most important (in order to shorten work time) or you can shorten your work window so that you must limit tasks to the important. Or just do both. In other words, choose to focus only on those things that get you the most answers right in the shortest period of time, then schedule them with very short and very clear deadlines so that they actually get done. Saying that you will spend 3 hours in one day working on test prep almost certainly ensures that you won't. If it's not mission-critical, then the unimportant will become the important, sabotaging your plan.
To maximize efficiency, it is also important to work just beyond your current skill level, but no further. Video game makers have embraced this principle for years; Make the game too easy, and no one is interested. Make the game too hard, and people give up. You want a challenge that is just out of reach, but still within sight. So you won't see me spending much time on concepts like matrices, logarithms, or obscure grammar rules until we've got the foundational stuff taken care of. It's just not worth the investment - bad trade. Instead, I focus only on what you are capable of doing, today. A funny thing happens when you do that, though - you start seeing the difficult problems differently, even without specifically working on them. That's the secret to the test - things that look complex aren’t usually complicated topics. Instead, the complexity stems from stacking basic concepts or techniques together.