To the OP:
From my own experience, I've found that instead of focusing on studying for a fixed amount of time, and then taking breaks at a fixed amount of time (e.g. studying for 30 minutes, then take a 15 minute break), I try to take the approach of "how much time in the day can I meaningfully use to lead a healthy life (exercise, sleep, bathe, eat properly, cook, do daily chores, etc.), and how much time do I have left over to study?"
Once I have determined how much time I have left to study, then I devote that entire time to study and see how much I learn the material I'm studying. Some material takes less time for me to learn than others. But I devote my full attention during that time. That could be 30 minutes, it could be several hours, but I devote fully to it.
Also, if I find myself struggling to understand the material, or stumped on specific subjects, then I might take a few minutes break (perhaps step out, do something else) as a means of separating myself from the material. This would give me both the opportunity to refresh my mind, and separate myself from the material momentarily to give me the needed "space" away to not become too stressed. Then I go right back in and study the material.
Keep in mind, though, that my approach may not work for you or for others. You have to try and see what approach to studying works best for you in terms of learning the material.