It depends on the turtle. One of my painteds has had the same aquatic plant in this tank for a year now. He breaks off pieces from time to time, but it's pretty much intact. It just stays in the tank, and I offer different veggies daily as part of his diet.
My RES eventually destroys any plant in his tank. I still keep aquatic plants in the tank, though and offer veggies. The veggies slow down his rate of eating the plants.
blind44, since you say your hatchlings nibble at the plants and veggies, I'd leave them in. I'd change out the lettuce (or whatever) if it started looking like it was going bad.
I keep a few small pieces of cuttlebone in the tank and replace as necessary. If they're being eaten really fast, I'd just add a few more the next day.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-