I have had occasions when my time is tight and I have gotten up and started getting dressed earlier then the full hour. Just letting her know that I had to go was not a problem. Usually though I would not say anything if we are both enjoying ourselves and she decides to stretch an hour. It all has to do with going with the flow of the session. I have never had a provider who did not know how to wind up a session.

that way she knows the time is up, and if she decides to continue that is her choice.

If you know you and don't mention it, even though she didn't stop it, you could end up with a reputation as a guy that goes over his time, or she could want paid for the extra time.  

If you book an hour and know the hour is up, mention it, it is honest, then whatever happens is her choice and you don't have worry or wonder about taking advantage of her time, or getting labeled as someone that does.

My ATF has small clocks...  I've noticed they move depending where we are in her location...  but she can not see the obivious bedside clock.  I've rolled over after & wow, we were past our time...  so I did mention it.  But she was in no hurry to have me leave.  If she has someplace to go, she'll say so.  
A good provider may track time through clocks, music tracks or just sense...   Time management is a necessary skill.  
Typically finishing the action is included... not starting something new.  You should say something if 10 minutes or more over...
skb

you need to respect theirs.

If you don't need to be anywhere, mention the time. If she encourages you to not rush, then it's on her, but I would make sure to offer her a tip. If you don't have extra cash, say so. If you're of a mind to return, mention that you'll return for an extended session.

If I can't compensate her, I would never stay past an extra 10-15 minutes. I would end the play on the hour and use the extra time to shower.

"I guess you better be going now" and she says, "Don't be in a hurry", then enjoy your extra time.  However, don't mention this part in the review since she may not offer extra time to all guys.  Also, a tip is in order.

So much so that I brought my own timer to sessions. I'd just start it when I knocked on the door, and it would beep with 10 minutes to go in the session so I'd have time to get dressed and get out of there before time was up.

I learned that some providers do not appreciate this; and one demonstrated her lack of appreciation by beating on the timer with a claw hammer.

LOL