Here is a simple formula today niksik told me for reading. Multiply the age of child with 5 if my son is 4 so it is 20 minutes and for daughter who is 8 it is 40 minutes.Her teacher recommeded 35 minutes or more for 3rd graders, but I seriously believe it depends on child's stamina.
PS> Icon bhaii I told 20 minutes since your child is hyper not 30.:p