Here's a complete guess, based on the information you've given. Don't blame me if it's wrong because the real answer is to water enough to keep the soil moist most of the time.
I guess you'd need to water the radish three times a week (because it's in full sun). You'll need to measure the amount of water. Pour water slowly on the pot when the compost is starting to dry out. When water starts to appear out of the holes at the bottom, then stop. That's how much one watering would take.