When is it time to euthanize?

I understand that it is a very difficult decision to make. Everyone has their own belief to the quality of life for their pet. I would just like to know peoples different opinions as to when they think it would be time for them to consider euthanasia? Could you take care of a diabetic cat who needed insulin for the rest of their lives, or a pet with kidney failure who would need fluids through a bag two days a week? What if your animal needs surgery to fix a broken bone after they are hit by a car, or just recovery after such an incident? Could you afford these things? Do you think it would ruin your pets quality of life? When do you think you would have to say"it is time"?