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.
Do five 20-second sets. If you have to pause between repetitions, that is ok for this days workout. However, do as many as you can in each 20-second set as if you were taking the PFT. Rest 25 seconds between each set.
Do 50 repetitions as fast as possible (without stopping if you can). Rest for 60 seconds. Do 30 repetitions as fast as possible. Rest for 45 seconds. Do 20 repetitions as fast as possible. Rest for 30 seconds. Do a maximum effort set.
Do as many crunches as possible in 2 minutes just like the PFT. Rest for 90 seconds and do a maximum effort set.
Training Sets and Final thoughts
A maximum effort set is the most crunches you can do without pausing. Once you stop, that set is complete. Sets on days 1 and 2 should be done in this manner, as well as, day 4 if you can. Days 3 and 5 focus on the maximum number you can do in a given time period instead of the maximum number you can do without stopping. If you will do this program consistently with the pull-up workout you will see fast and effective improvement in your PFT score. The key is consistency. The workout does not take much time, so stick to it.
USMC Crunches: PFT Rules