The Gemara (Sanhedrin 99a) explains Bamidbar 15:31 “Ki Davar Hashem Bazah” – “because the word of Hashem he mocked … he deserves Kares” as “this is one who could learn Torah and does not learn”.
The Rambam (Hilchos Talmud Torah 3:13) brings this: “And also he who could learn Torah and does not because of laziness is included in mocking the word of Hashem.
In Shulchan Aruch 246:25: Anyone who could learn Torah and does not, or learned and left to Havlei Olam is included in he who mocks the word of Hashem. The Rema adds that it is forbidden to speak idle talk (Yoma 19b v’Debarta Bam – speak in Torah and not idle talk). The Mishnah Berurah 155:4 writes that Talmud Torah has no limit and its obligation is all day whenever there is open time, as it says “the Torah should not be forsaken from your mouth” (Yehoshua 1:8).
The Yerushalmi adds: He who sets time for Torah and only learns at set times is breaking the Bris.
In Shulchan Aruch 2:4 we find the Halachah that one who forgets one thing from the Torah … because of not properly reviewing transgresses the Torah commandment of Devarim 4:9 which refers to forgetting Ma’amad Har Sinai and includes forgetting any part of Torah.
In Menachos 99b Ben Dama asks R’Yishmael: I, who learned the entire Torah, can I learn Greek wisdom? He answered that the Pasuk says, “The Torah should not depart from your mouth and learn it day and night” – if you find time which is not day or night only then can you learn Greek wisdom.
In Binyan Olam chapters 6 and 13 Bitul Torah is discussed at length. There is also qualitative Bitul Torah. Learning without full effort, concentration and comprehension is Bitul Torah (Shulchan Aruch ha’Rav, Talmud Torah 2:4).