depends on the boat, how it's weighted, how good you are, the type of tricks, how big you're taking them.
If you subscribe to Wakeboarding magazine, they had a good article about rope length. You want to hit the wake where it's clean, so the rope length will be different for each boat.
If you give us more info then maybe we can help. But on average, I'm guessing for you 55 - 60' since you're riding at a relatively slow speed. For example, I ride at 22 mph. Generally the faster you go the farther back on the wake it will start to wash out.
(Message edited by sdboardr99 on July 02, 2002)