More isn't better, only BETTER is better.
Holding a plank for an extended period of time.
I think a standard plank, and the ability to hold it, are invaluable. A plank done properly will teach you how to engage your core in the right places - tucking the pelvis, creating the hollow body position, etc. Your ability to sustain that position with correct tension is a worthwhile skill that is directly transferable to other more important exercises.
But how long should you hold a plank for? There is a common mindset out there that encouraging prolonged periods - 2min, 5min, 10min... and well, the world record is approximately 8 hours. The conventional mindset is that we should attempt to hold a plank for longer and longer periods of time; with longer equating to better.
Now if you know me, you will know that I say "more isn't better. Only better is better." In this case, more time isn't better.
There is always a case of diminishing returns. You will reach a point of optimal return in almost any endeavour (specifically fitness related). After that you are working harder and harder for less and less return.
The question should be, "is there a BETTER way?"
Why are you performing a plank? Is it to teach core engagement, and correct (hollow body) positioning? If so, holding a plank for 1 minute correctly will teach you all you need to know. If you can hold a plank for 2 minutes, or anything beyond that, you need to ask yourself what *more* are you getting by continuing to hold that plank?
If you can hold a plank for 5, or even 10 minutes, what is the first minute doing for you? The answer is nothing - it's only getting you to a place where your plank is becoming challenging; so why not do something to make it challenging from the beginning? Adding weight is a great way to do this - instantly, from the very first second in a weighted plank, you're are working hard - hopefully to your capacity. Why waste 5 minutes just getting yourself to the point where you are working hard - and therefore creating the training stimulus you are after - when you can skip that 5 minutes and go directly to the point where you are properly engaged; working hard enough to create enough training stimulus to improve on your current ability?
Adding weight is one way. Another way is by performing a more advanced version of the exercise. If you can hold a plank for more than 2 minutes, you really need to be looking at more advanced versions - or even more advanced exercises.
If you are "pretty good, or advanced" at holding a plank, why wouldn't you move on to more advanced core exercises?
If you can sit down into a deep squat position and stand up again, that's great. But will you then say that you want to get REALLY GOOD at sitting down and standing up again, or would you progress to more challenging versions of the squat - adding load or performing a more difficult version, something more challenging on the system?
It's like I say about running - the best reason to run is if you enjoy running. If you are doing it to lose weight or get fitter, there are better ways of achieving those goals than running. If you enjoy running, then run. If you don't have a preference, and your goal isn't specifically running, then chose something more optimal. If there is a better method, use the better method.
I have spoken about the plank, about squatting, about running – but it is not about specifics, it's a concept that you need to apply in your training. This is what I do, I teach conceptually. If you can understand the concept, you can apply it to anything.
More isn't better, only better is better.