I think you answered your own question-  ymmv - it's going to depend on the provider- look at her reviews- see what other reviewers say about a particular star you are interested in- but really hard to answer as a general question

I Would Imagine

Like the guys said, read reviews, but even then...every rendezvous is different.

My question to you is, why would you expect a porn star to extend her rendezvous past the time you've paid for?

There are a lot of factors that go into how strict they are with the time:  
   - Do they do their own scheduling or does someone do it for them?
   - How much time is there between appointments?
   - How much time does it take for them to get ready?
   - Do they like you, are they enjoying your company?
   - Are they in this for the fun or the money?
etc.

 I've had some porn stars end things exactly on the hour. I've had some that said once you cum, you're done and I've been shown the door in 30 minutes, or 45, or something else less than an hour.  I've had some that were not clock watchers at all and we ran over a few minutes until their agent called.  I've had some who let me stay an additional 30 minutes or an hour because we were enjoying each other (not just the sex).  And I've even had a few who let me stay a couple of additional hours.  My favorite of that ilk was the legendary Anna Malle - we met up for the hour which turned into 90 minutes, we then went out to eat (she paid and then we went back to the room for another 90 minutes (or so - I lost track).  

So you can see that there are all sorts of possibilities and there are just too many variables to give you any kind of answer other than YMMV.

You're much better off asking about a specific porn star than asking a general question like this.  Even with a specific star, YMMV as she might treat you differently than she would treat me, or someone else.  See the list of questions at the top for possible reasons.

The higher the price per hour usually the more strict they are about their time. That's been my experience.

Best advice I can give you:  

If you book an hour, expect an hour.  

Don't be rude or greedy by expecting her to provide beyond the booked time. Just like you don't expect a therapist/plumber/driver/maid/gardener/musician/lawyer/any other per-hour service provider to do so.  

If you think you need/want more than an hour, book the extra time.  If you want her to be professional about her service, you need to be professional about using it.  

Then, and only then, if she feels like spending more time with you, and if you are able/willing to do so, go ahead and enjoy the gift.

Thanks for all the advice.