I also pay at the start. Mine puts her computer down beside her and turns to her calendar to verify when I come in next. She said her internal clock tells her when time is up and then she'll look at the clock right about time.
Funny thing is for the last 7 months that I've been seeing her regularly, I thought my sessions were 50 minutes. I don't watch the clock and noticed that she sometimes goes over. When she didn't, I would get irritated that she ended early or on time. I got off track on my co-pays and got a bill stating 45 minute sessions. So, she had been going over every time. I told her that all her clients would probably appreciate her adding 10 minutes to her sessions. She said the longer the session, the longer it takes the client to talk so she's found her time frame is best. If they know it's just 45 minutes, they are more likely to dive right in.