How Much to Feed
This will depend a bit on your individual turtle. A good rule of thumb is to feed as much as your turtle will consume in 15 minutes. If you feed in a separate container, it is easier to monitor how much and how fast your turtle eats. It is important to offer a variety of foods at the same time -- don't just give your turtle its favorite items.
As an alternative guideline, another common "rule of thumb" is to give meals that are roughly the same size (in total volume) as the turtle's head and neck. Still, you'll need to monitor your turtle's body condition and adjust if necessary.
Keep in mind, that if you are feeding strictly by these rules, feeding every day might be too much, and every two or three days might be too little. You will likely need to adjust the amount you give your turtle. The best guide is to monitor your turtle's weight and condition, and with a little experience you will get to know better how much your turtle can eat and stay in good condition. You also do not want to let juvenile turtles grow too quickly.