Err, the only answer is when it needs it. You will have to get a feel yourself for when that is, if you can't just check it frequently (like daily).
I'm currently on something like a 5 day for most of my paphs, but one is typically a seven day between. They're indoors, and in east and west windows, and in coconut husk based media. I'm using aeropots (net pots with a transparent clear pot around them) so that tends to produce greater humidity than a pure net pot would...and slower drying. My temps are about 70F now indoors, but RH is about 50-60% as yours is. But each plant is kinda an individual and not really tied to the calendar
Periods of rainy cloudy weather increase the time between waterings..and sunny breezy days (when I open the windows) decrease them.