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I've noticed that most other turtles will start eating right after the owner feeds them. My turtles are too scared of me to eat right away so I leave it in the tank. I feed them 6 sticks apiece, a romaine lettuce leaf and half of a baby carrot once a week. They pick at their food all day, taking little bits here and there; and I'm afraid of taking their food away before they've had their fill.

Is it ok to do this or should I have a strict cut-off time limit?

I think that's fine as long as they're cleaning it up.
But you'll want to watch to make sure they're both getting the same amount. Often one will end up bullying the other and getting all the food.