Well, there is no real amount of time that sex "should" last. Some people are fast and others are slow, and that's ok.

Personally, I am fairly quick to orgasm for a female, so 10-15 of minutes of sexual activity is usually all that I need.

Well it depends. I personally like a lot of making out beforehand; mutual masturbation and what not. My fiancee has incredible control over himself during oral sex, so we can go at that for 15-30 minutes after making out. Then there is the actual sex which is normally about 5-10 minutes. All in all, we normally spend about an hour for the whole event. We can go longer though. Actual penis-in-vagina times vary from 5-30 minutes depending on what we're doing. If there's a lot of changing positions then obviously the actual intercourse lasts longer. If we're just doing a quickie then say, 15-30 minutes.

I agree with this.
I think it really just depends on the day/night and what kind of mood we are both in. Sometimes we will kiss and cuddle and make love and this all takes an hour or more. Other times it's shorter 20-30 min, and then there are quickies, lol.

Many of my girlfriends in the past all wanted sex for at least one hour and sometimes pushing it to two. It wasn't difficult for me, since I actually loved each one and it was easy for me to be hard. It all depends on the orgasm period, i.e. how long it takes til the girl cums, and how many times she wants to orgasm. But performing oral sex on the girl helps a lot. If you know what to do with the tongue that is.