It depends on the tube and whether or not you are running both the plate and filament voltage. Most units have a standby, that keeps the tube warm, but not on. This can extend the tubes life quite a bit (assuming it's in standby 24/7 and on much less). It also depends on the tube, many NOS tubes can last much longer than current tubes. Mullards are noted for their long life (although I've found them to be a bit "tubey" sounding). Siemens are also long lasting and I really like them--very transparent sounding. If I remember correctly ARC charges a real premium for you to buy tubes from them, so I would recommend a different source. Upscale Audio, Tube world are two I've used with good results.
How can you tell when tubes are shot?
I have an ARC REF1 and ARC REF Phono Preamp as well as assorted solid state gear. I leave everything on 24/7 and am wondering if the tubes just quit working (ie: get really noisy) or whether there is a gradual decline in their performance? ARC makes recommendations on who long they should last but I have been running the REF1 steady for over 18 months and haven't noticed anything.