I think clockwatching happens far more with 'managed' WGs than indies. Managed can range from fully pimped to the girl paying for a booking/answering service. Of course you are going to be short changed if someone has booked punters on the hour, every hour. They are bound to be wanting to wrap things up 20 minutes from the end.
Over my last few meets with indies I've had a mixture of setting an alarm for the full time when the action starts, or going with the flow of the meet and not really worrying too much about going as much as 15 or 20 mins over. I don't mind the alarm going, after all they have clearly advertised that you pay a set price for a certain amount of time.
In my experience, as long as they know that you're not going to take the piss, girls who allow themselves adequate time between bookings don't get worried by going a few minutes over. If this does happen I don't normally take a shower as I think this is then pushing things too far and when you next meet they'll remember to allow time for you to shower, so they will want to cut down on any over time.
I was caught out by the alarm once as I had timed my climax (almost) too perfection and was just about to blow as the 60 minute alarm went off, and as she lent over to switch it off (not to terminate the meeting) I simultaneously shouted out "leave it!" and blew my load - it rather spoilt the moment.
As already stated, many use music to time the different length of meets, and I'm convinced that one chose the playlist accordingly, so that if you worked with the rhythm of the music you climaxed at about 55 mins and I suddenly found myself fucking to the beat of the music. Perhaps if you hear "Flight of the Bumble Bee" 20 mins From the end, you know she is trying to short change you!