No set number, when the top of the soil starts drying out (not all of the soil though) then give them some water. Obviously, watering too much or too little will hurt the plant - but there's a lot of room in between the two. It's pretty easy, you'll get used to it quickly.
Mine tend to need water every 3 or 4 days, but yours could be different. Lots of factors can change it - temps, humidity, plant size, type of soil/medium, etc..