The going wisdom seems to be that doing a little bit of something good is better than not doing it at all. Doing 2-5 minutes of meditation, 20 minutes making music, 5 minutes of working out, 10 minutes of yoga. But some isn't enough if you're hoping to get the rewards that come at the end.
Some is definitely more than nothing, but I've found that the benefits from many practices are nonlinear. That they're actually tiered in segments of decreasing time (logarithmic). So you might be in a particular phase for the first 20 minutes, then 10 minutes, then 5, then 2. When exercising, it's common knowledge that we have to push our muscles past the point of comfort until we start to cause them to tear.
Yet when meditating, doing yoga, or doing our creative practice, we don't ask where the point of fatigue is. During the first 5 minutes of meditation I may find settle into a nice feeling of peace, but the work starts after 10 minutes when I get bored and want to get up, and in earnest after 15 minutes when my mind wanders for the 10th time and I am actually challenged to accept my thoughts as they are.
I write morning pages everyday. The first page is drivel - sloughing off the thoughts from the top of my mind. The second page I usually work something out or develop a thought or concept, perhaps about a creative project, a relationship, or money questions. The third page, often half way down and without fail, is where I have my breakthrough moments
It's not that the first page isn't valuable. A little is indeed more than nothing. But I want a little of what happens on the third page. To do that, I have to slog through the first two pages. This is why I commit to three pages every morning. Not to an hour. Not to what feels good. Three full pages. Because the benefits I'm seeking happens on the third page.
What does this look like applied to other practices? To meditation, it probably means that 5 minutes isn't the same as 20. I may already understand that because when I do 20 I feel better than 5. But perhaps it's time to start thinking that it isn't because I did 4x more, it's that I did infinitely more of the last 5 minutes.
Taking the 80/20 rule, it's in the last 20% that we get 80% of the benefit. Not the other way around. The trick is knowing how long you have to go before you get to that 20%. At some point we get to diminishing returns. How do we know where that is? Ask those that have done it a lot before. No one around? Do it longer and then ask yourself.