Funny story: Before my first time I was worried about not lasting long enough, because that's the big cliche about virgins. So to increase my potential staying power I went out and bought this brand of condom made of extra thick latex. It was advertised as an endurance booster or some such. So the day finally came, I put on the condom, and my partner and I then endured the most tedious half hour of intercourse in the history of man. Eventually she asked what the problem was, I told her about my special condom purchase, we both had a good laugh, and off that sucker came. I followed shortly thereafter and a good time was had by all. The moral of the story is you shouldn't let concerns about how long you will last psych you out. You might do better than you expect and most partners of first-timers aren't going to have any out sized expectations about your performance ability. Just enjoy.
10 seconds. If you do the research, you'll find an overwhelming number of accounts where guys said their first time lasted about as long as it took to get it in. A mixture of over-excitement and over-stimulation. Or the opposite--they wilt because of the pressure and over-excitement of finally getting to this point. Time doesn't matter the first time, or the fifth, etc. You build up. It's like asking how many strikes you're going to get the first time you bowl. Chances are the first time you bowl it's going to be a gutter ball. Over time you develop the skills to make strikes. Over time you build your average. Same with sex. If you're with a woman who's going to judge you poorly because of ineptitude and inexperience, you need to find another woman.
I had the opposite problem - it took forever for my brain to get out of the way and let my body climax. Fortunately, my partner was just as understanding about that as most women will be about cumming right off the bat. To the OP, just because a man orgasms doesn't mean the sex has to stop.