Could you tell me what is the half life of risperidone?

Depends. 20 hours after oral administration and 2.9-6 days after injection into a muscle.
It varies. The average half-life for oral risperidone is 20 hours. (A small subset of individuals metabolize the drug rapidly and have a ~3-hour half-life.) In intramuscular risperidone, the half-life can be between 3 to 6 days, which is a function of it being released over an extended period of time.