It depends how long you have been practicing meditation for. If you are new to it, sitting for long periods of time, even for 30 minutes, can be too much. You might just not be used to it, you might not be sitting correctly, you might experience more back and knee pain, and a few other things that could cause you to burn out quickly from the practice and just stop it.

In comparison, meditating for even just 5 to 10 minutes a day in the beginning has quite a few benefits, the main one is that you don't burn out and get tired of the practice so quickly, instead, you build it up and make it a long term practice. Other than that, it helps build stamina, strengthens the muscles required for sitting, and allows you to slowly get into the mindfulness part of the practice with less stress.

So I would say that sitting for 15 minutes, or even 5 to 10 minutes can be more effective than sitting for 30 minutes, 1 hour, or more. Even for the more advanced and well practiced meditators, sitting for such long periods of time is not necessarily more beneficial than a short session.

I was always under the impression that meditation means to stop thinking. That the purpose of meditation is to be able to stop your mind from thinking. Now I know that this is not the case and that this is not the purpose of meditation.

But this is a very common thought. I hear people say quite often that they weren't able to quiet their minds while meditating so they stopped because it's not for them. Or that meditation makes them more upset because they keep thinking and keep trying to stop thinkin which makes them think more, and on and on.

I understand them, because I had the same idea of what meditation means. But meditation is actually pretty far from that.

It's not to stop your thoughts, it's not to fight your mind, it's not to sit in stillness with a blank mind. It's simply to be aware that you are thinking, and to calmly, bring that awareness to the breath, or to what ever your object of focus is, but to not fight to focus on it.

It should not be a battle, a task, or a chore. It should simply be to become aware. From this awareness while meditating, you will slowly become more aware outside of meditation. And this is where life starts to change.

The longest session I have done was a 3 hour long session that included about 10 minutes of walking meditation every 25 minutes or so.

I did this once a week for a few weeks, and the more I did it, the more I got used to it. The sitting was pretty difficult, but here are a few things I noticed:

1. After doing this for a few weeks, the sitting still felt as difficult in the end as it did in the beginning, but, it was far more tolerable, which made the whole session easier.
2. Many times I found myself waiting for the bell to ring that indicates to begin the walking meditation. This would usually replace even my string of wondering thoughts as It was so difficult for me to keep sitting, I was sort of waiting for the relief of the bell and anticipating it.
3. Other than the above point, thoughts almost never stopped, but, that isn't the point of meditation. I'm noting this just to not give the impression that there was any attempt at stopping the thoughts from coming in this practice.
4. Sitting back down after the walking meditation felt like a sort of reset, even though the walking was just about 10 minutes and the sitting before was so difficult. The sense of a reset really helped make it tolerable and less difficult.
5. There were moments of "clarity" in which I felt as if my mind was in a dream state. There were still thoughts, but, they took on a slightly different form, I wasn't attached to them or focused on them, but I was just aware of them as they were playing in the background in my mind. This usually didn't last long, but it was an amazing feeling and experience when it happened.
6. I felt a great sense of accomplishment after each session, no matter how the session went.
7. There was a sense of calmness after each session, again, regardless of how the session went and if I was full of thoughts the whole time.
8. Doing this type of practice in a meditation session with other practitioners made all the difference. I don't believe I would actually be disciplined enough to do this whole session on my own. The presence of others really played a huge role in keeping me going.
9. It wouldn't be fair to not point out the physical pain I guess, so yes, there was pain in my back and in my legs, and they would tend to fall asleep. I would shift around and switch my legs around to wake them up and prevent them from falling asleep. Overall, it's not pain I would be worried about or that would prevent me from doing this again, it's more of an uncomfortable type pain, and it got easier as the weeks went on.
10. The beginning of each session was easier on the mind than the end. In the beginning I felt more in "control", not in the sense of controlling my mind, but just being less on auto pilot and having less thoughts. The end of the sessions were far more chaotic for me, thoughts were out of control, I would also find myself following the thoughts story line a lot more, rather than bringing my awareness back to the breath. I would engage in the stories.

This was part of a month of constant meditation in which I meditated twice a day almost every day, and went to lectures, so it would be pretty hard to say what impact this specific part of the practice had on me for that time and after. I can say that those days were quite peaceful, less stressful, and just overall more mindful.

Overall I would say it was a great experience. It was without a doubt very difficult, but very rewarding also. I personally jumped right into it, starting with daily meditations twice a day, including this 3 hour session once a week, but this may not be the best approach to this, maybe some would benefit from starting with much shorter sessions, or just the hourly sessions once or twice a day.