First time rule.

23 minutes.
That is all the attention I will devote to the woman the first time we have sex. Not including foreplay, but actual in/out thrusting. Why not more?

Ten minutes is too short.
Twenty, sounds good, but I am not a fan of even numbers.
Thirty,meh, too long, and I am out of shape.

It also solves the question:
Keep going?
Should I stop?
Is she enjoying it
Is she moaning because she really likes it, or is she trying to get me off.
I wonder what her head would look like on my dashboard.
Etc.

Twenty three minutes and done. Thats all a woman gets to get off the first time.
Didn't orgasm? Not my fault, ample time was given.
After that, comfort levels are usually dropped to something managable and I can get a good feel of what she is into. Maybe she likes to be smacked around and choked with a electric heating blanket cord? (details later) You never know.

Now, I will adjust accordingly. But when I am out of the loop with a complete random this rule has never failed me. Coincidently enough, I have a clock built into the bed of my headboard which leaves little room for error. If that fails, I always have the CD that is playing in the background. Once it hits track 7 I know it is time to start closing shop.