I don't wish to be contradictory towards what Fr. Anastasios said as far as going from midnight to midnight without food or drink, but, as others have pointed out, work schedules, family schedules, dietary needs etc. will either make such a rule easier or more difficult to follow. Fr. Anastasios' point about going from Vespers to Vespers is subject to cheating because people can always celebrate Vespers earlier or later to make their fasts shorter. I'd establish a rule, whatever rule, in consultation with a priest, of course and then follow it. But remember, and this is important, that the Publican was justified not because of his fasting but because of his humility. It is always a means to an end. At the same time, it is a vital discipline in the practice of Orthodoxy. there are so many threads about that on this board. Just remember that if you are new to Orthodoxy, you can overdo fasting to the point of seriously causing harm to yourself. the best way is to ease into it. Just mho.