ALWAYS ON. Solid state or tube. Always.
I never turn mine off. It's an Audible Illusions M3A. It's known as a "tube eater". I had the same tubes in it (Amperex JAN) for 5 years. The JAN's are still good! I just took them out to listen to different tubes.
Think about it. Professional studios leave everything on constantly. Nothing heats up then cools down then heats up .... These expansions (heat) and contractions (cool) cause a shorter life.
To test this, leave a light bulb on. Isolate it from all vibrations. (You never thought you'd be using cones, cork, sorbothane, ... for that, did you?) That light bulb will last longer than any other light bulb from the same pack.