Actually I was being quite conservative in the length of time tubes can last. For example, several years ago I bought an original McIntosh C11 preamp which had it's original 1962 tubes - and they still work fine. In my power amplifiers, the output tubes routinely have lasted me 10 years, and the input tubes have lasted at least 15 and still counting.

The length of time output tubes last is completely dependent on the setting of the bias current. If your amplifiers have user adjustable bias, you should make sure that it is never set to more current than the manufacturer recommends. In many instances, the current running through the output tubes can be set slightly low (something like 10-15%), with remarkable increase in the life of the tubes - in some instances, decades.

So no, tubes are definately not something you have to change every year, or even every 5 years.

I'm aware of what some high end manufacturers say, essentially that you have to change output tubes every 6 months or somehow they degrade to the point where they sound "bad". That is pure BS.