Often you hear that you don't need more gear for longer trips. That makes sense to some extent but on a trip of several months or years I would definitely bring some extra comforts. A bigger tent and a chair come to mind, possibly a full-size laptop, a 5 oz electric razor... I still think I could keep it under 40 lbs. On a really long trip you might be doing shorter days so the extra weight might not affect you as much. Most people will also lose weight, which will at least partially offset extra gear weight.