I tend to feed my turtles daily in summer, feeding them veggies one day and dry food or worms the other day. In winter I only feed twice a week and mostly dry food, because it is cooler, and the population density in my tank is up. Full is, when the turtle slows down eating. Stuffed is, when the turtle cannot get any more food down, even if it tries. But don't do this often.
Captive turtles tend to be fed too much meat.
Young turtles need more meat than adults. If you are feeding a perfect diet, no supplements are necessary. Calcium Turtles need calcium to build healthy shells and bones. A cuttle fish bone in the tank will be nibbled on by most turtles and supplies extra calcium. There are also calcium supplements specially for reptiles.