For potatoes to grow properly they need to be watered every 2 to 3 days.
In the beginning you can water the potatoes every 4 days but when they begin growing you should up the watering schedule to every 2 to 3 days for optimal growing of the potatoes.
It takes from 60 to 90 days to be ready to pick full grown potatoes and during that time water them every 2 to 3 days by soaking the soil with some water.