It really depends on the individual. Some people have better endurance and some don't. In this case it doesn't matter. To my mind, people who get tired fast are just as worthy of progress as people who can practice for several hours straight.
What matters is this: what do you do when you get tired? Do you close the instrument lid, put down your music and do something else? Or do you keep on playing regardless of how you feel? Or perhaps do you take a rest, drink a glass of water, take a short walk or stretch, do some other short tasks and then come back to practice for some more? (That's the best solution, I think).
Of course you are aware that it takes some 10 minutes just to warm up the fingers and feet. So if all you practice is 10 minutes, then it will be difficult to accomplish much in organ playing because in reality, you are just repeating previously learned material and not learning anything new.
If on the other hand, you are constantly learning something new every day, then you don't have time to repeat what you have learned in the past during these 10 minutes.
From my personal experience I can say, that people usually get tired in doing one task after about 30 minutes. So if you practice for 30 minutes, then you get about 20 minutes of real practice after your body feels warmed up. That's not bad. You can still practice effectively in the long run if you spend 30 minutes daily on the organ bench.
By the way, don't forget that our practice is not limited to the organ only. Here is an article worth reading.
All of the above thoughts apply when you are practicing for your own pleasure only. If you are preparing for public performance and have a due date, then the situation is entirely different. Here other people depend on you preparing on time. Then you need a very effective practice plan with different length of practice sessions which would ensure you would reach your goal on time.
Because having a real plan means you are going to have a much better chance of reaching your goal, I highly recommend you think about your goals in organ playing and set some deadlines even if nobody else knows about it and even if you play just for your own pleasure and not for performance in public.
Please note that I'm talking about the scenario when you can choose the length of your practice session. A completely different case would be if you only have 15 or 30 minutes a day available for practice. Then it would seem you don't have much choice of how long should you practice. Even then, I recommend reading this article which perhaps would give you some ideas how to find more time for organ practice during your day.
So you can see that the practice length is a far more complex issue than simply playing until you get tired. It also involves planning and goal setting. Therefore, the time you spend practicing depends on answering these 6 questions:
1. What do you want to accomplish?
2. Why do you want to achieve this goal?
3. When would you like to reach this goal?
4. What is your plan of action needed to reach this goal?
5. What are the daily steps to fulfill this plan?
6. What is the time required to complete the daily steps?
If you think deeply about these 6 questions and you write down the answers on a separate sheet of paper, then you will find the ideal length of your practice session very easily after answering the last question.
By the way, if you would like to explore the topic of goal setting more fully, then the works of Zig Ziglar would be a great help.
What about you? How long are your daily practice sessions? Share your thoughts in comments.