1/2 h is nothing you should be worried about. Besides, showing some stamina and not shooting your loud within the first 5 minutes is rather a great plus than anything else.
Yet, just like with anyone else, it is your dick and you rightly want to have some sense of control. Besides, you may want to 'reward' a good sucker dude, who might not be precisely the marathon man, if you are catching my drift.
Here, practice makes perfect. It always takes 2 to tango. That is why you want to learn how and when to delay and/or speed up your orgasm. Just like with any other form of sex, it should not be only about you. So, the more experience you gain, the better you get.
It is a good fun to practice, when it comes to BJs, isn't it?
SC