I haven't looked at specifics on chain in a number of years, but am familiar with modern galvanizing techniques.
You should look carefully that the chain is hot dipped galvanized. There is no equivalent. There is a newer technique that creates thick zinc coatings, it's called mechanical galvanizing. It is often marketed as equivalent to hot dipped, and in some applications is darned close. But it is not equivalent for chain.
Hot dipped galvanized chain should give years of use, and when it is getting closer to loosing it's galvanizing, will show some rusty surface. However, even in that state it will still be protecting the chain but is a good indicator that you can start looking to get that same chain re-hot dip galvanized and go on for many more years.
dj