There is not really an set amount of time. They are a consumable: like tyres on your car. It’s the amount of hours they have on the road and how harshly they treated that determines when it is time to change them.
The usual things to look out for are:
- crackling
- hum
- microphonics
- blowing front panel fuses
Tube tester can monitor the health of tubes, consider building one from a kit such as the muTracer3, The uTracer, a miniature Tube Curve Tracer / Tester.. Without test equipment you can only really respond once you notice an issue.