How to find out your own concentration limit

Only a few people can concentrate on something for a long time. Especially whilst doing something that is not so pleasant, it is very hard to keep up the concentration. Attempting to push your concentration while you force yourself to stay seated doesn't work, or if so, very rarely. It very often ends with you cursing yourself, because your thoughts are meandering. This is clearly not a solution.

Every person has his/her individual concentration limit. If you can't keep your concentration for 30 minutes, try it for 15 minutes. And if it is impossible to keep it for 15 minutes, try it for 5 minutes. Sometime it will work and then you can gradually raise it.
But you should reach this limit. It's better to stop working as soon as you lose your concentration than to keep on working without concentration.

After each "concentration phase" (and even if it is very short), it is important to take a brake and to relax yourself. During this break you can let your thoughts wander. The break shouldn't be too long, and don't do things like reading or similar activity. Relax! It is better to look out of the window for 2 minutes. Find out how long your break has to be.
And: It's not enough just to change the subject. Just do nothing, that's much better.

Sometime you have to come to an end. 6 hours per day concentrated working is normal, afterwards the concentration tapers off. Although you can train your concentration and in extreme situations you can learn for more than 6 hours, you should keep an eye on your limits, and try to realize where your limits are and lower your own aims.