Length of meditation

If I meditate too little, I don't have spiritual energy throughout the day. If I meditate too long, I can't maintain my aspiration.

The problem is that you are going to extremes. Sometimes you overeat and you cannot digest what you have eaten; sometimes you don't eat at all and you feel weak. When you don't meditate enough you feel miserable, and when you meditate beyond your capacity you feel worn out. But you don't have to pull or push. You only have to discover how much capacity you have. If you meditate for only five minutes, then naturally you will not be inspired throughout the day; but if you meditate for an hour or two, you will strain your capacity, and your body will revolt. So meditate devotedly and soulfully for twenty minutes in the morning and twenty minutes in the evening, and in the afternoon if you have a chance you can meditate for five or ten minutes more. That will be ample in your particular case.