How often do i need to change the tubes in my amp?

i bought a Peavey ValveKing about 8 months ago, and when I did the guy who sold it to me said i will need to change the tuubes once in a while. how often do i need to do that? and if i dont, what happens?How often do i need to change the tubes in my amp?
The average life of a power tube that is used three to four times a week should be approximately 18 months to 2 years.



The average life of a pre-amp tube if used three to four times a week should be approximately 3 to 4 years.



Of course, that estimate will vary from the kind of amp that you use to different degrees of playing styles.





David