Using a motor as a generator: How fast does it need to rotate?
A typical motor will behave as an ideal motor in series with a certain amount of inductance and resistance.  At all times, the rotational speed of a motor will be a certain multiple of the applied voltage, and the torque will be a multiple of the current flowing through it.  These relationships are bidirectional, so whether something acts like a motor or a generator will depend upon whether relative directions of the applied voltage and the current flow.

Based on the above description, I would estimate that your the voltage on the ideal motor will be roughly 1 volt per 1500rpm (look at no-load speed divided by rated voltage).  I'm not sure whether the stall current is measured at 2 volts or 6 volts, but the short-circuit current from a motor spinning at a given speed will be roughly equal to the stall current of a motor driven at the open-circuit voltage that would correspond with that speed.  To get maximum power output, you should draw less than half of the short-circuit current,.