This depends on several factors.
First, the surface finish.If it's bright annealed, pickled, or electropolished it won't discolor until there is an accumulation of salt on the surface from local conditions.
Polished surfaces will begin to corrode in spots within days.
Second, The cleanliness of the surface. If the surface is rinsed by rainwater often, this will prevent discoloring possibly indefinitely. You control this by making sure surfaces are exposed to rain and don't have recesses, shielded places, or horizontal surfaces on which evaporation can concentrate salt.
