The longest plank challenge to me sounds like the weight lifting equivalent to 1 rep max. I understand the challenge, but have always found the N x T approach substantially more effective as part of a core strength training routine.
N= # of planks
T = time
I’m sort of in the middle of my plank fitness, but I do 6 x 1min with 1 min rest (i.e. it takes 11mins to complete). The next progression (that I’ve recently started) is to add 5 seconds to each plank within each workout (i.e. 1st is 1:00, 2nd is 1:05, 3rd is 1:10, etc., such that the 6th would be 1:25). Initially they are done with the same 1 min rest between planks, then later I allow the rest only to get me to the top of the next minute (i.e. recoveries become shorter). Eventually I’ll work back up to 6x1:30 on 1min RI (where I was last year). Going longer doesn’t seem like a good use of time for me as I find it better spent on other core exercises.