Here's a link on feeding pellets:
http://www.redearslider.com/pellets.html
There's other good info in this general area too, on feeding not just pellets but other things as well.
You'll see the recommendation is to estimate how many pellets your turtle's head (not including his neck) would hold if it were hollow, and feed that much every other day.
Alternatively, if you want to feed your turtle half a head full of pellets every day, that would be the same amoun overall and would be fine.
Turtles will eat just about as much as you'll feed them whether they need it or not. That doesn't mean it's good for them.
Turtles need fresh green leafy things as a regular part of their diet along with the pellets. They can have this every day.
Turtles also need cuttlebone in their tanks to provide calcium. You can just leave some pieces of cuttlebone in there and they'll nibble on it from time to time. Read up on how to propare the cuttlebone for the turtle tank.
Things like shrimp are very high in protein and shouldn't be given very often. Once every few weeks or once a month as a special treat is just fine.
Enjoy!