As Eric said, there are no clear boundaries for determining this. But I will describe what I used as my personal barometer. As you're studying and working problems, are there specific problems that when you see them you think to yourself, "AAHHHH!" while on others you think, "whewwww, it's one of these." Does this repeat itself over and over again when you see these same types of problems? On the practice CATS, when you see to these same problem types do you react the same way? If so then you have probably discovered a strength and a weakness. The fact that you are glad to see a certain type of problem because you're very confident when faced with those problems but terrified when you see another type is a good indication that you need to spend less time on the easy topic and instead use that time on the hard one.
Let me qualify that though - if you are only aiming for the low 600s, then you can continue focusing on your strengths and still make a decent showing; however, if you are shooting for 700 range, you cannot have any clear weaknesses. The topics that you face are randomly given to you so if you are unlucky and see many from a weak category then that can seriously derail your chances to hit 700.
Another way to easily locate a weak area is to compare your hit rates over time. If you hitting 80% on RC and SC, but only 60% on CR then it's pretty clear that CR is a weaker area where you should spend more time. Note that as you see more and more difficult problems you can expect to see your hit rates drop. If you see a significant drop while other areas stay consistent then shift your focus. For example, my hit rate for DS was in the high 80s up until the last 50 problems in OG11, then plummeted to 57%.
Anyways, the underlying message is that it's really up to you when you feel it's right to shift your focus and it depends on your target score.
