I'd turn it off. Wastes tube life. However, some tube preamps run the tubes at very low power, so that can litterally last a long time. Some questions to ask yourself, do you notice a difference in sound quality if left on for days at a time? Or after a hour or two, things are warmed up and sound fine? Are you running stock tubes that are cheap to replace? Or expensive NOS? Turning tubes on and off a lot can reduce life span.
What I do is on weekends, when I listen a lot, leave it on. During the week, goes on and off...
What I do is on weekends, when I listen a lot, leave it on. During the week, goes on and off...