I'm more of an "every couple months" diffuser cleaner, but your Diffusers will last longer if you take better care of them!
How many drops in the diffuser?
Small room: 3-5 drops
Larger space: 4-6
Large open space: 6-10
I don't suggest doing any more than 10 drops total for a large and open space. You can determine how strong you want the smell to be.
These will vary depending on the potency of your oils. For example Lemongrass and Cassia are very strong smelling oils so you can use less. Start with the smaller number of drops and add more if you prefer.