I'd say good for years.
They key to the sound is to find one with original speaker and original output transformer. What absolutely needs to be done for safety, great sound and reliability is -
- re-cap all the power supply
- clean and preserve potentiometers
- remove "death cap" (can kill you someday, no joke!!!)
- replace bias cap and put variable resistor for precise bias setting (on fixed bias amps)
- install 3-prong grounded power cable, remove polarity switch wiring
- cut the high voltage "protection" circuit from speaker cable (causes hum, can kill your speaker and output transformer, can shock you with +450V)
Other things worth doing (if needed): replace the seal between cab and head, re-fit the speaker frame, clean or re-cone the speaker, change the speaker cable to 1/4" connection and good speaker wire
What you should NOT do - change caps/resistors in the signal/channel circuit - they have their own sound we do love. And 99,99% times they don't need replacing. Also, don't put 6550 tubes and stuff like that, you'd kill the OT.