I very rarely can see a clock of any kind while I'm fucking, but I do have a general sense of how long I've been doing it. I've always been a believer that the longer I last, the more pleasure I get and the more intense my orgasm will be. In general, the longer I last the more pleasure the woman I've been with has experienced as well.
I certainly notice when I don't seem to last very long, despite my being aware of my arousal state and pausing or shifting position to create a 'break' in stimulation. Sometimes that works, and sometimes it doesn't. I try not to think about it as that can sometimes cause me to reach the point of no return and cum even though I wanted to go longer. Sometimes I notice that I'm not getting that 'feeling' and try to put that thought out of my mind and continue fucking, hoping I didn't jinx myself. Sometimes that works and we continue on with many more minutes of pleasure, and sometimes it doesn't.
On extremely rare occasions where I've lasted more than 20 minutes of direct penis-in-vagina sex, I get tired and the woman generally is more than ready for me to be done as well. 30 years ago, lasting 20-40 minutes of actual fucking, switching positions multiple times, finally collapsing after having a nice orgasm was much more common than today. Both being younger, full of lust, and fucking at the drop of a hat had a lot to do with that.
I'm still just as horny today, but life gets in the way. I'm hoping that when we finally retire, that I can chase my wife around 3-4 times a week!