R. Eliezer says: for the days of the prophet Jeremiah. For when Jeremiah said to Israel: Why are you not studying Torah, they said to him: How will we feed ourselves? He took out the flask of manna and said to them (Jeremiah 2:31) "O generation, see the word of the L-td, etc." Your fathers, who studied Torah, see how they were fed. You, too, if you study Torah, the Holy One Blessed be He will feed you of this.