I was told by a Surly dealer that, starting with 2015 models, Surly now will pre-treat the inside surfaces of the frame and fork tubes with rust inhibitor (such as Frame Saver or T-9). I could not find any substantiation for this on the Surly website, so I'm not sure whether this is true. If true, then, if subjected to wet weather, the Surly frame would probably last longer than a non-treated steel frame. You should be able to treat the frame yourself, but this requires stripping the bike down completely for the procedure.
My previous steel bike lasted me 13 years as an all-weather commuter. Eventually one of the chainstay tubes rusted inside out, all the way through. I treated my current steel tube bike with T-9, I wonder how long it will last as I continue riding in the winter in salty slush.