Just how much do you spend on tubes per year on average ?

I have 50 tubes across my system and have replaced zero.  My system is on more than off.  Sometimes I keep it on for weeks at a time.  Don't let tube replacement intervals prevent you from buying tube components.  

If the system wont get used much, then you may never need to replace them.  

A lot depends on type of tubes employed and how hard the tubes are driven.  My system happens to use tube types known for long life and my amps don’t push the output tubes at all.  This is VERY important as some of these tubes are quite expensive.

Well, I use Mullard EL34 XF2 power tubes. They are known to last for a long time but rarely longer than 5000 hours, as I understand, even if NOS and pushed gently. So, I think, in my case I should expect $500-$600 a year to lose on all tubes. My $450 monthly electric bill in New England makes me uncomfortable but the tube bill will not.

I no longer have anything that uses power tubes. I currently only use Dual Triode tubes (12au7, 6922, 7308 and…) and they last 9-15 thousand hours, so to answer your question, very little.

My wife grew up with her dad's system crippled by tube failures ruining her music. She bans them from our system. It's not about the money. She just really hates tubes.