I've been adding coolant for almost two years now. It appears that when I do mostly highway driving, the loss is much, much less. When I do add the coolant, I notice that it does take a while for the system to suck down sufficient coolant, i.e. it may take a minute to gurgle down then I add more.
Mind you, the car does have 194,000 on it. I don't want to spend a lot more money on the car but would like to keep it running. Thanks so much for your input. Your comments have been immeasurable.