Turning on and off the equipment is more harsh on the tubes than leaving them on. Leave them on if you listen quite often. If you plan on not listening for more than a few days, then turning the unit off. If you leave your unit on 24/7, the tubes should be replaced 2-3 years. Occassional listeners like Kinsekd could probably expect his tubes to last longer. The best thing is to always have a spare pair available. Try compare the old set of tubes to the new set and see if there's any sonic differences. If it is pretty obvious, then that means the old ones should be replaced. If you use NOS tubes, it is better to stock up some now, as I've seen prices drastically going upward for them.
