It's really impossible to give a number in gallons of water. There are just too many factors involved: soil composition, drainage, water holding capacity, atmospheric moisture, air temperature, etc.

In general, a thorough soaking once a week unless there is substantial rainfall is better than 2 to 3 times per week. If the leaves tend to curl even when temperatures decrease overnight, water is needed. Overwatering is more difficult to detect, but if there is good drainage and you stick to the once a week soaking you should not experience any problems.