Haa haa! I can honestly say I have never looked at the time when I'm having sex but I know if I have gone on for a long time or not.
The more you fuck the better fucking shape you'll get in But just dont wide open the whole time and when you need a breather roll over and make her do some work. I have yet to meet a chick that can go as long as a me (any guy). When you make the chick do some of the work you dont get the "dude doesnt last long enough" because they just cant hang and hit it like we can Take a break, get a drink of water and go back at it
^ Yeah, when a girl tells you to stop, that doesn't necessarily mean you've 'worn her out'. She might just be bored. If this is happening consistently, and you keep hearing "gawd, you're taking too long", you might have some things to consider other than stamina.