Hello Shreekant,
Congratulations on your purchase, please do not hesitate to drop by here and ask any questions as you build.
Tube life is dependent on quite a number of factors, some of them are:
1. How hard you're operating the tube (plate dissipation)
2. How much the applied heater voltage exceeds the minimum rating
3. How many power cycles occur within a given time period of operation
I have found that something like an American 6SN7 operated at a conservative operating point and running constantly will last for 4+ years, that is 35,000 hours.
On the other hand, I have seen Chinese 6L6's operated at a conservative operating point last about 4 months in constant operation (~3000 hours).
A 6DN7 in the SEX amp will fall somewhere in the middle.
-PB