How long does "galvanic corrosion" take?

I recently had a plumber replace a 40 foot corroded section of galvanized steel irrigation pipe with PVC. The pipe connects directly into the main water supply to the house, with no shut off valve in the event of a leak. Upon inspecting what he had done, I noticed he had threaded a copper section of pipe into the existing galvanized steel connector. All that appears to separate the copper from the steel is a thin layer of white tape and some paste material. I called after he left to ask about this, and he told me some story about a rubber gasket separating the pipes. I see the other end of the copper section (which is only about 3 inches long) is attached to the PVC through a gasket and connector, but the threaded end is definitely touching the galvanized steel. If I do nothing, how long will it take for the copper to destroy the steel? The steel is already corroded on the outside, but is still strong. Should I insist he replace the connection? This is a large plumbing company that "trains" an army of young men to go out and do their work. I don't think the people doing the work are licensed themselves. I don't want to be unreasonably demanding, but I don't want to deal with another leak in the main line any time soon either.
Thank you in advance for any advice,
Tina

pipe

As a practical matter, if the pipe already has corrosion it will probably fail from that long before galvanic action is a problem.

Part of the equation is the actual composition of your water - how reactive it is (the pH and other things). Hard to tell for sure.

I'm just a novice but you noted something else that seemed odd to me: don't you need a water shut-off for the house. It's not just for leaks, you need it to turn off for repairs, faucet replacements, new water heater etc.

Would a sacrificial anode made of magnesium connected to the galvanized pipe work? Magnesium is more likely to ionize and corrode.