How deep is the water? Is it deep enough for a small/gentle filter, by any chance?
What else is in the tank? Substrate? That might make a difference on how you go about the water changes. Any plants? Anacharis is soft, doesn't need to be planted (can float freely in the water), and does no harm if the hatchling decides to taste it. It also helps with water quality.
What kind of hatchling turtle is it? What size? Some turtles are more delicate and require more exacting water quality parameters. Is there a land area or anywhere the turtle can haul out of the water?
Do you feed the turtle inside the tank, or in a separate container? If the turtle accepts it well, the separate container cuts down on the amount of uneaten food, debris, and turtle poop in the tank.
Consider getting a turkey baster at the dollar store and designate it for turtle/tank related use only. Use it to remove uneaten food and turtle poop to keep the water cleaner for longer.
I know I'm not answering the question right off the bat, exactly, but like I said, it depends...
I like to do partial water changes one time a week. If its 13 gallon of water I would do 3-4 gallon change every week. With aquariums even if you have a good filter the stuff that getting filtered out still is in the system just trapped in the filter where bacteria is eating away. The water will be clean but it's all just the same water, adding fresh water and taking out part of the old water will help keep the water cleaner and healthier