So I guess if you book an hour, but come back in 55 minutes. It's YOUR fault for not maximizing the other 5 minutes, or does it get carried over?
No, this will be governed more by what happens on the lesson, it may be that an extra 5 mins will not achieve much,and worth finishing early to save the cash or that an extra 5 minutes will be worth doing to finish off what is going on in the lesson, it could also be that traffic at the airfield causes you to land 5 minutes or so over. Just work on the basis that the lessons will be around an hour initially and you will pay for what you actually fly, it could even be that lessons end up at 45 mins or less or over 1:15, though either would be unusual apart from the nav bits near the end.