Yes it is indoors in full sun. It's in a plastic 160z cup with 1inch cut off from the top, and filled with dirt almost to the top. The temperature varies between 60 and 68 degrees Fahrenhiet. I'm not sure what the humidity is, but I don't think it's very hight since it's indoors and it's still pretty cool outside (40's). I never knew this was going to be so complicated.. :P
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.