That's hard to say. You might get 1 month, you might get 10 years. But I think the norm would be 3 years of a lot of use and you may be pushing the limit.
One thing you might check is to see if the tube has become "microphonic" . With the unit cranked up and warmed up, flick the side of the tube and see if it produces an audible noise through into the signal path. In essence it has become a microphone and is not much use anymore.
The problem is that even brand new tubes can be "microphonic". Thats one of the reasons high end tube gear can cost more. They reject a lot of tubes trying to find good ones.
