[Amps] GI-7B's life

[Hsu]

I have some Chinese GI-7Bs(Cninese renamed FM-7F,I have call the factory, they told me its quality is same as Russian's), I want to do a HENRY TEMPO 2001 tube convesation,origional tubes is a pair 8874.
  Although I have bought a pair 8874, but I think it is too expensive to me($500/pair).I'm thinking to replace 
a pair 8874 to  a pair GI-7B , I have never do it.But I think it should not be a problem  to me,I only want to know
its life in 700-800W HF/VHF/UHF out put level.Could someone can tell me its life? So I can cauculate how much  GI7B I should be reserve.
  Thanks in advance!
           73! Hsu