How long does Mobil1 last if not used, but installed...

Oil never wears out, But it gets dirty, water in it, corrosives from the combustion process from the gasses that leak by the rings. Moisture from cooling and heating and cooling and on and on. Oil has gotten expensive as evidenced by discussions in other threads on this site. BUT, oil is the cheapest thing you can put in that engine to prolong the performance that you expect from it. Don't jeopardize your investment to just cheap out on a few ounces of motor oil.