Your Diffusers will a last long time if you take care of them! I just wipe mine out with a dry paper towel every 1 or 2 uses.
Standard diffuser recipes run roughly
500 ml 15-20 drops total
300 ml 10-15 drops total
100 ml 3-6 drops
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.