Actually, that'll probably be "chock to chock", not "choke to choke" (light aircraft engines usually don't have them, certainly C152s don't). This basically means that it'll be from when the wheels start moving for taxi, until they stop at the end of the flight back in your parking spot.
As for how to make it exactly 60 minutes - it's a talent that flying instructors have, I've never quite fathomed it, but they do it with unnerring accuracy.
£99/hr for lessons in the UK by the way, is pretty good value. Personally I don't like C152s much, they are deadly boring - but to be fair, they're an excellent training aeroplane.
G