Midrash for Bava Metzia 58:17
אבל לא ילמוד בו בתחלה ולא יקרא אחר עמו: ורמינהו לא יקרא פרשה וישנה ולא יקרא בו פרשה ויתרגם ולא יפתח בו יותר מג' דפין ולא יקראו בו שלשה בני אדם בכרך אחד הא שנים קורין
Our Rabbis taught: If one borrows a Scroll of the Torah from his neighbour, he may not lend it to another. He may open and read it, providing, however, that he does not study [a subject] therein for the first time; nor may another person read it together with him. Likewise, if one deposits a Scroll of the Torah with his neighbour, he [the latter] must roll it once every twelve months, and may open and read it; but if he opens it in his own interest, it is forbidden. Symmachus said: In the case of a new one, every thirty days; in the case of an old one, every twelve months. R. Eliezer b. Jacob said: In both cases, every twelve months.