According to PDF from Janssen
For those patients stabilized on a fixed dose of oral risperidone for 2 weeks or more, the following conversion scheme should be considered: Patients treated with a dosage of 4 mg or less oral risperidone should receive the recommended dose of 25 mg RISPERDAL CONSTA once every 2 weeks. After an adequate trial at the recommended dose, some patients may benefit from higher doses of 37.5 mg and/or 50 mg of RISPERDAL CONSTA. Patients treated with oral risperidone doses higher than 4 mg per day may be considered for RISPERDAL CONSTA doses of 37.5 mg and up to 50 mg once every 2 weeks, after a trial at the recommended dose of 25 mg once every 2 weeks.