I water when the containers are lightweight and soil deep down is close to bone dry. Watch and wait as the containers filled to the brim with water settles out. If you're watering more than one plant, the drainage speed will show you which ones have better drainage, which may hint at improving soil mixes in the future or lining the bottom for drainage. Quick drainage means they were thirsty and that oxygen is getting to the roots.

Personally, I wouldn't even look at a calendar for watering and instead let the plants tell you when they're thirsty. I've gone as long as 5 days and as quickly as two days between waterings.