I don't know how old it can get, but I saw somebody writing about a 60 year old Hoya.
As to how to keep it going, I have a simple way of doing it: watering only when the soil is dry at the bottom of the pot, sometimes after 2-3 days of dry soil. I have never fertilized it. Mine is only 5 years old and was started from a leaf taken from my mother's plant which was 10 years old at that time.
Water ammount depends on how big the plant is and how fast it uses the water, but you can get a general idea by looking at the leaves: leaves start turning yellow when overwatered. If they keep being green, but they are thin and start turning/twisting the edge, it means it was deprived of water and should be adjusted gradually to a higher ammount.
For propagation I have used individual leaves that can be easily be removed from the stem of an adult plant. I have just put them in plastic cups with general purpose soil bought from store - nothing fancy, and watered from time to time. If you try this, keep the new cuttings in a well ventilated place to avoid fungus development in its soil. If you don't have a well ventilated place, let the soil dry for one day before watering again.