3 things must be true if you want to sing high for longer periods of time:
1) you must use the correct technique. Incorrect technique is fatiguing to all the muscles in your larynx. Improper vocal "posture" may even cause injury in the long term in a similar way that improper posture while typing on a computer can cause carpal tunnel syndrome in your wrists. So yes, continuing in this manner may hurt your voice.
2) you must have the endurance to sing for longer periods of time. In order to sing, your laryngeal muscles must "tense" (in a proper way, mind you). Your muscles are only capable of tensing for so long before the glycogen storage in your muscles are depleted. After they are depleted, any further exercise is counter-productive and may, in fact, hurt your progress. The specificity principle applies here. If you want to sing for a long period of time, you must train endurance.
3) you must be mentally capable of maintaining proper technique. You can only focus for so long. Once you start losing focus, your technique will suffer, especially if your technique is not so strongly pre-programmed into your brain. If you want to dribble a ball down the field or court, then you must be mentally capable of focusing on your technique. If you lose focus, it leaves you open to faults and the opposing team can take the ball.