For a moderately strong women, I would suggest anything from 45 lbs and under. If you find this to be easy still for the amount of reps prescribed, there are a few ways you can make the exercise more difficult:
1. Adding in instability through a bosu ball or foam pad
2. Slowing down the movement to a 3 count on the way down and 3 count on the way up.
3. Holding weight away from body so that you are forced to use core to keep back straight and upright.
Remember that technique is the most important thing in doing any weight lifting exercise. Full range of motion in the correct movement should come before any increase in weight.