OK. Final count is in. I go through a liter in 5 years. My sister goes through a liter in 10 years. So, we need 3 liters for each decade.Experiment done. 30ml took 8 days to go through. So 3.75 ml per day.
I had originally estimated based on 3 ml/day at 18 mg/ml. Now, it's 3.75/day at 12 mg/ml. That would be a liter every 6 years! But, I'll estimate at 4.5 ml/day (5 years/liter) to allow lots of leeway for waste.
Now I just have to wait to see how fast my sister goes through the 90 mls that I just gave her. I may not need to buy any more after all.