This is a popular training program for the crunches part of the PFT.
While it is a good program to help you get to and past 100 crunches, remember that it crunches are a one-dimensional component of fitness and are not a substitute for a full core workout. OCS is much more than just the PFT.
Do three maximum effort sets. Rest 60-90 seconds between each set. As soon as you stop (even for one second), that set is complete.
Pyramid day. Begin with 20 repetitions. Each consecutive set should increase by ten until you can not complete the required amount without pausing. Then do a maximum effort set. Rest 1 second per crunch of the previous set.