I tried using a countdown clock app on my phone but it seems pretty tacky.  

...there will be a clock somewhere in the room. Just glance at it once in a while. Check what time she arrived if it's outcall (she comes to where you are). If you go where she is (incall) she will have a clock. She will be aware of the time.  

and gals need be good at discreet time management.  The only way it might become your responsibility is if she provided outcall to your location.
Gals often use music tracks timed.  I've had situations where I could not see a clock but after several visits with her, noticed a small clock was discreetly moved to what ever room we played in...  on a mantle, on a book shelf.  
I have occasionally come up for air & having lost all track of time, had to move to see the bed side clock (she had turned the glaring LED bedside clock so it wasn't easy to view).

i usually do not worry about the clock..my dick knows when the time is up..

That's her job.

Unless you have somewhere to be or if you have to go back to work, it's the provider's job to keep track of time.
If you both are having a great time and the conversation is flowing, she won't be too concerned about leaving so quickly.
She may reward you with extra time if you were a gentleman and if the chemistry is there, so don't shoot yourself in the foot by excusing yourself on time.

are clicking and the vibe and energy is good, then I am not that worried... We will be done when we are done...  

I personally am a "clockwatcher". I guess because prior to joining TER I was seeing agency ladies and a few times we'd get the "call" because we were running over. Now, even though I've been seeing indy ladies I still have this worry/guilt about her time and if I can't see the clock I will ask her "how are we doing on the time?" Kinda dumb of me - no? Because when I think about it, she may perceive this as me being "tired of her" and wanting the session to end. (BTW- these are all incalls)

They were enjoying my time (not tired of me), but wanted to make sure  we were still good on time and that it was ok that we were going over.

I'm amazed at how many guys don't think first before writing some of the stuff they write. Not just about the OTC they received but also about some service they received that is not standard on her profile or previous reviews. I guess its more about "self-bragging" like "I was so great she gave me an extra hour" or "she gave me CIM". Maybe they don't realize the potential problems they are creating for that lady.

and I will mock hurt in my voice and say Wow! I will have to sadly let gals know in my review that that you are a clock watcher AND you are rushing me  
lol