I read a book once that gave very simple advice for most indoor plants: Watch them. They'll ask for water when they're thirsty.
This is good advice. Creating a watering schedule based on an arbitrary number of days won't work for most indoor gardeners. A plant's need for water depends on too many factors: humidity in the room, light source and intensity, quality of the soil, time of year, and general health. Instead of trying to put an unnatural schedule on a natural plant...pay attention to the philodendron itself!
Keep an eye out on your plants every day. Just a quick walk-by to say "hello" or "good morning" will let you know who's thirsty, who's "caught a bug," and who needs to be turned toward or away from its light source.
For watering: check for drooping leaves (though if their space heats up during a particular time of day, the leaves can droop from too much heat), thinning stalks...and then stick your finger in the soil at least an inch down--farther if you can--to check for dryness. (Also a good way to tell if your planter needs topping off with a layer of fresh soil.)
To get the most out of keeping your philodendron well-watered, be sure your planter has good drainage.