I agree as well with most of the responses on here. One minute is a sufficient time to hold a plank. With that being said, I try to get clients to contract every muscle in their body while they are doing that one minute plank (with quality form). Once a client can do a plank for a minute, you add a progression to make it more difficult. Lifting a foot up, reaching a hand in different planes, putting them in a TRX, plank on medicine ball or stability ball. Any way to increase the challenge (within reason). One a client goes past a minute, it is just for fun to see how much stronger their core strength has increased.