How long can bare wood can be exposed to weather?

You mean without degrading or weakening? It depends on the type of wood, and the weather/climate it is exposed to. For instance, cedar and redwood can be left outside unfinished for decades and still be sound. Other woods, like common construction lumber do not fare nearly as well. In dryer climates wood lasts longer. In tropical climates with high humidity, wood tends to rot if not coated with a protective paint or stain.