This has been discussed several(many) times.
For me personally it is door to door, or Knock to knock.  Either incall or outcall.
It comes down to the simple fact that you are paying for an hour of the providers time. If 1 hour isn't enough time to have the experience you are after ask if she offers hour and a half experiences.

You pay for an hour, you get an hour. How you use the time in between is between you and her.

Of course, in the interest of customer service and getting return business, the provider may strategically schedule some "slack" time for greetings and getting ready to leave.

Or she may not.  I have seen providers go to great lengths to cut short the playtime.  They might make you wait in the parking lot until 10 minutes after the hour, then small talk for another 10 minutes, then spend another 5 in the bathroom and broom you out the door right on the hour.  

Miami Companion girls were notorious for this crap, partially due to their unreasonable schedules.

I won't return to a provider or service that does business like that. Ever.  I won't even look at MC girls ads anymore.  

multiple hours, I don't concern myself with when the time starts or ends. Most often, my dates are for outcall, and I prefer to let the lady manage her time as she sees fit. I've only been burned on the time a couple of times, and as it turns out, it was just as well. Those were not among my best experiences. On the other hand, I have rarely had a lady get up and leave "exactly" when the agreed upon time was up. Most have been quite generous.

There was a well known escort in Vegas who brought a kitchen timer with her, so that she wouldn't loose track of time.  We scheduled for 90 minutes, and when the timer went off, she was way beyond stopping.  We went two and a half hours, and I could see why keeping track of time was a problem for her.  Of course, I was a lot younger then, otherwise I would have been done, long before the end of our scheduled time. LOL

Assuming both parties are on time, I would once the two of you are together and she is ready to pay attention to you.

I sometimes call the girl as when I find the room number and when they picku up the phone I say "I am here" and then I knock.

That gives me an exact time in case she tries to short clock me.  I try to look at the clock, too.  Otherwise, I don't consider it a go until I am her focus, which may mean she has and has counted the donation and called her contact.  Then I look at the clock and say the time out loud.  

If I walk in and she kisses me and we are off to the races, then the hour starts from moment one.

If she has some bs burn up the clock routine, I am not paying for that nonsense.  My hour starts when she starts escorting for me.  

Here is a good rule of thumb for someone who will short clock you: she calls herself a courtesan....