Uh, this is actually a rather difficult question to answer, as it varies from person to person. In general, what Bates recommends in his books is that you only continue exercises that give some sort of benefit, whether in the form of relaxation or improved vision. However, it seems to me that the latter is not a good indicator of "working ability", since when I "check" to see if my vision has improved or not, it invariably gets worse. Therefore, I think it is better to ask yourself whether you feel relaxed or not, whether the exercise "feels" right, etc. I think it's a matter of trust - if you have that gut-feeling that it will or won't work, go with it and see where it takes you. Remember that you can always come back later if you want to.
Sorry, but that's all the advice I can give you. I'm sure other people have their own opinions on this topic, so hopefully, they'll contribute to the discussion as well. Good luck!