I am a newbie and I am wondering when the clock starts. Is it the second she enters the room?

the clock starts when you are in the room with her.  Suggest you read the self help center, and come back here if you have more questions.

LLAP,
Swim

...I think we all have our biases when it comes to punctuality and time. My suggestion is to not try to keep track of the time down to the second.  

All these ladies are different... some are very punctual and organized and some are not.  

Let's take for example the most common, which is the hour appointment. You'll find that you'll have some that will only last 50 minutes and some will last 75. it really depends on the connection, the chemistry, and both of your schedules.  

I think if you're too anal about it, you'll miss out on the important part, which is to relax and enjoy the charms of your companion.  

Just my dos centavos...

Should be the moment she walks in the room, if she is late it is on her.

If you are coming to her, some start at appt'd time whether you are late or not. Some start when you enter. Some won't see you a minute before your time, some will. Everyone is different. Some won't even see you if you are running quite a bit late. Some guys think it starts after he takes a shower, others don't assume that. Also, if you pay for x amount of time, never ever assume you get to stay longer. No matter what her reviews say, because some idiots do put they stayed longer, don't think you get the same. If she lets you, it is on her, or she will tell you you can extend for x amount. She shouldn't ever make you leave before your time though.  

I think ("think") for the majority of highly reviewed gals, it starts once either of you get there... especially if you are not running all that late.

if you see an agency lady,you get there and once inside you shower. does time start when she opens door/

for the shower before and sometimes after (about 5 minutes for each). But you should never assume that going in the first time you use the agency. A previous poster got it right when they said everyone is different on this. Always assume time starts when you walk in the door and you need to be out on the dot of time being up. Let them tell you once you get there if you don't need to hurry. This for in calls of course.  

Personally I would be the same way on out call. I'd assume time starts when she walks in the door and I'd expect her to leave on time. If she was late I'd expect to receive my full time or something would have to be worked out. If she stayed a little overtime I wouldn't be pushing her out but neither would I do anything to impede her leaving. I'd expect to compensate her if I asked her to stay longer.  

I am a clock watcher. I always have been. I am relaxed about that now with independents I've seen often and agencies I am familiar with but first time with either I am on the dot unless they say otherwise. Even then don't take advantage be cognizant of their time and be prompt about arriving and leaving

Unless, you are told otherwise, assume that you need to leave the room by 59:59:59.

Years ago, I worked with a very busy, 'reputable', mid-price ($300-500) agency. I had appointments back to back to back. 4PM, 5PM, 6PM, etc. If one client went over or arrived late, it threw the whole schedule off. That got REALLY stressful.

I worked for another agency that allowed 15-30 minutes between appointments, but in a pinch, they would schedule back-to-back.  

Even if she is allowed time in between sessions, she still has to (hopefully) shower, and straighten up the room before the next dude comes up.

So in summary, realize she might have other appointments scheduled right after you leave. Next guy might even be downstairs already. Unless she tells you 'No rush', keep it moving!

Should start at the appointment time & it's up to you to be there on time.  Yes, it can cost you a few minutes to get to her room.  Likewise if she comes to you, she should be on time.  If she's late, it starts when she arrives.  
Quality gals should allow enough time between appointments to wash / shower / rest between appointments.  Sometimes the ideal doesn't work out through out the day...  but she can't give you her best if she's tired.  
THe time should end promptly.  Gals must be good time managers.  While clock watching is generally bad, gals use different tricks to help with time management...  hidden clocks, music tracks, or just a good sense of time.  
If she's relaxed about time, that's on her & ok but you should not abuse the privledge.  If you want a shower after as well, you need to allow time

I'm wondering if you book a 1 hour session and decide, before it's over, to pay for additional time what is the standard policy. Let's say you like the girl a lot, enjoy her company, and just want additional time to be with her. Do most of the providers agree?

...she would be able to say yes or no immediately in most cases. An agency provider would have to call in and get it approved.  

As long as another appointment isn't scheduled which would interfere you should be good to go. Though independents could have other aspects of their life that might make it impossible to accommodate your request. You would need to have the extra cash on hand.