I hear copper stays FOREVERRRRR. That's why it isn't a good idea to convert a fresh water tank into a salt one....unless, it was yours and you know it didn't have copper.
I believe you only really get copper from using the HOT water due to the copper piping but not from using the cold water even though its still coppper. Also wouldnt running activated carbon remove this? When I filled my freshwater tank from the faucet I never had issues with copper (no snail or invert deaths). Then again I only ran the cold water and used a heater to bring the water temp up to target.
I have a client who used copper in his tank about 5 months ago....tested today and there are no traces of it. His tank was never treated either. None at all