It can sometimes be difficult to quantify. When does a generally better service for a regular, maybe for that reason or maybe partly just because you get on pretty well, become something 'extra'?
I've not done a lot of punting over the last couple of years but, previously, I was very regular with a touring girl who was certainly relaxed about extra time - but, then, I would often be there very late in the day and we both talk a lot, so it's not like it was hour after hour of 'free' services.
Having said that, it wasn't uncommon for her to initiate something after time was up - but we are talking about a girl I saw roughly once a month for a few years and I think we both did well out of it.
Other than that, another fairly regular girl offered me an overnight for the cost of a second hour during another late punt - which was the best value I ever had. However, that was a one-off, so I assume it was just something that made financial sense to her there and then, rather than a favour to me!