We learn in Avos "Be not like servants who serve the master on condition of receiving a reward, but be like servants who serve the master, not on condition of receiving a reward", how the more so someone who wants a promise or pledge from H'shem.
pg 457
Nefesh Hachaim
One may think that taking one part of Torah, Mishna, and learning it with fervor and fierce dedication is preferable. Yet we learn that it is not that way and setting out to learn as much as one can is advised.
Gate Four, Chapter 2 pg 137