Just for kicks I thought to see what test equipment people have and how often it is used?
I have some cardboard strips (still!) and the api master FW test kit.
Also kh & gh test kits. As well as iron and potassium.
Lastly a digital ph pen to check the ph controller on the co2 injection.
I'd say I test nitrates and phosphate every 1 to 2 weeks. Check ph and kh/gh about every 1 or 2 months. Potassium rarely (although it involves 5 bottles so makes up for it in time maybe )