Life span of 5U4G tube?

>>You can probably expect a 20 year lifetime from major-brand NOS 5U4Gs.<<

Perhaps.

Life expectancy depends primarily on the number of thermal cycles (on/off) and hours of use.

It could last 5, 10, 20 years or even more.

Wow, thanks. That seems longer than a NOS 6SN7 tube lifespan.

If your 5U4G is new or NOS, measure the B+ voltage at the HV first power supply cap. I assume you have read the usual safety precautions about dealing with high voltages.
Make a note of B+ DC V.
If in the future your amp sounds wimpy, check the B+ voltage again. A drop of more than 20 Volts as compared to new specs means that your rectifier is weak (though I have never seen it happen).
If you suspect your rectifier to be weak, compare it to a new tube.
You can probably expect a 20 year lifetime from major-brand NOS 5U4Gs.

There are no mortuary tables available that I know of.