Is this copper to galvanized fitting hidden inside the wall or is it easy to get to? If its easy to get to, don't worry about it. When it leaks, if ever, replace it. If it is inside the wall then you can estimate about a year or two if you use the shower a lot, maybe nine to twelve months if not used too often. The corrosion caused by the dissimilar metals is a chemical corrosion that allows the softer metal copper to give up ions to the atmosphere, and allow oxygenation to rust away at the galvanized protection of the harder steel. The friction of a lot of water flow causes a slight bit of electrical interraction to occur at the same time that the copper ions would normally be released, and they stay put better. Without a lot of flow, the ions can release at a faster rate and corrosion can get rampant. Sometimes though, the corrosion actually seals a pinhole leak that might arise as a result of ionization. A worrysome bit of news, eh? There's a good reason people hire LICENSED plumbers to do their work. Good luck.
: I have recently replaced the piping in my shower. The old pipe was galvalized and the new was copper. At the time, I did not know about the crosion issue with dissimilar metals. I used a threaded copper female fitting and screwed it into a galvalized elbow. Does anyone know how long this will last before I need to worry about replacing it?