I recommend to work with your child until your child does not want to learn anymore. Follow your child’s lead. I recommend about 15 minutes a day. I read/learn an average of 10-30 minutes a day with my children. I let them stop when they want to stop. They love to read and so I never push them to do more than they want.
My children always have had a Reading Reward Chart and they received up to 3 points a Reading Lesson based on their concentration and effort. After they received 20 Reading Points they got a toy from a dollar type store.
The important thing is that the child is getting focused learning time. It is not the quantity of time you work with your child but the quality of this time. I do not feel it is wrong to reward a child for working hard to learn to read but it is up to each parent on whether or not he/she believes in rewards for Reading.