The frequency I have used recently is zero of these in the months leading up to the test. In lieu of sit ups, I did planks for one minute at a time, every day, never even as much as ten minutes of planking a day.
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When I did them I did three sets three days a week, worked up to 30 or 40 in a set, short rests. That got me into the 80s for the 2 minute test, head on floor to elbows on knees, fingers interlaced. If doing planks works you could consider doing sit-ups only once a week to practice gaming the movement, and use the planks as the primary training. Doing it that way would probably be better for your spine.