New tubes every year...or every five? ?

I may get thrown off this forum, referred to an ear specialist and sent to Circuit City to replace my system, but it recently occured to me that I did not change the tubes in my recently dumped Jadis JPL pre amp for 5 years! There is no on/off switch on this pre amp but there is a "stand by" switch which was always on. I would say I listened to the system for only an hour or so a week. Since trading my Apogee stages for Martin Logan SL3's about 3 years ago, I rarely listened to the system and found it uninvolving and a little fatiguing and bright. It didn't necessarily sound bad -- I thought I just needed to buy a new D/A or go back to LPs. Is it possible that my very light use and the standby switch would give the tubes such a long lifespan or would a change of tubes have dramatically improved the sound? Maybe I should have kept the system?!? Oh well, more fun to buy new stuff...