There is no limit to what there is to know of the Torah, and even if a person were to live for as long as Elijah the prophet, he would not succeed in knowing even a thousandth of what there is to know. In Heaven the person is punished for what he could have know but did not try to learn. And he is asked on the Day of Judgment even about the most esoteric aspects of Torah. Now, the Holy One certainly does not expect His creatures to do what they are incapable of doing, and He does not punish them for things that are not in their control. Rather, it all depends on what a person is capable of learning. It is, however, correct to learn first those areas of Torah that are most necessary, such as the common halachos, works on Mussar, Fear of Heaven and proper service of Hashem. Afterwards, he should continue and try to learn something new every day, some piece of knowledge that he did not know yesterday. By doing so, he will acquire perfection for his soul and he will not lose time—that most precious commodity—which he could have used to acquire Torah knowledge.