Depends on what you want to call "marine" environment. For most of us, out guns are kept in enclosed closets and away from the elements. As long as they're well oiled, they'll never rust. Add Marinecote into the equation and it will last a long time. Mine stayed in an oiled sleeve in a closet for a few trips (Mossberg Marine), so I really don't have any long term experience, but I conceal carry several types of .45's all the time aboard and have never had an issue with rust. Again, they stay well oiled and are wiped down regularly.
I've also talked to several marine patrol guys that carry 24/7 aboard. While they obviously take good care of their weapons, they don't experience any rust problems.
