Ideally, you can learn your body and understand what causes you to climax. That way, you can decide when to orgasm, whether it be in 2 minutes or 2 hours. I find that much more satisfactory then thinking, "oh ok, well I'm at a 10 minute limit, so if you want more sex after that, then sorry too bad".
If I'm not tired and the girl wants lots, I will go for however long she needs it for. If we are pressed for time, then it will be a different length. It's all about understanding yourself physically and mentally so you can enjoy sex for however long you want (or need be) without ever worrying about orgasming too early.
A Japanese study showed that when stimualted continously the average male lasted 180 seconds plus or minus 60 seconds. So unless you are stopping a lot, most guys don't last that long in the real world.
Well, you should strive to learn to control yourself so you can last how ever long is necessary to give her what she expects. As was said, it is difficult for most men to thrust continually and last longer than a few minutes, at best. Most men need to slow down, stop and/or withdraw for a bit (in the guize of changing positions) or think of physics equations or something to last longer. Note that some women, especially those who can't orgasm vaginally, may not want you to last long. They would want you to take your time going down on them or engaged in foreplay, but they won't mind if intercourse only takes just a few minutes. Other women who can reach orgasm vaginally might need 10 to 20 minutes of hard thrusting to get there, and that can be challenge for many men. But, if you are with a woman who needs or expects it, it is something to works towards. Some men will have a quickie and then they will go a second round where they will be able to go for the long haul. But, the main thing to consider is what SHE wants, and many women simply don't want intercourse to be a marathon. So, ask her.