As much as his/her little heart desires! It is very challenging to overfeed these guys as most will stop eating when full. As long as you are offering a healthy diet they should be allowed to eat to their hearts content. Most tortoises can consume a pile of food about the same size as themselves daily, so that is probably a good starting point. If you notice left over food or if s/he finishes it all quickly you can adjust accordingly.
Thank you I actually put some St. Augustine grass in the enclosure so he can eat when he wants. What about Timothy hay? That and alfalfa hay are all I could find and they say alfalfa is not good.