Quality of life for an older dog. Could I have your opinion on this matter?

I have a almost 16 year old Pomeranian/Chiahuahua. He has congestive heart failure and has been on Lasix and a few other meds to help deal this issue. He has been on the medication for 3 months. It seemed to be helping him. He hardly coughed anymore.

The last couple of nights, and now all day, today he’s been coughing.

I am worried about his quality of life. How do you know when is when..