CRT life expectancy ,whats your opinion ?

[Tube TV]

I read this in a book printed in 1964 it says that the average life is 3 and a half years some more or less .
I have got far longer that this on many old televisions , most with original CRTs and how many hours on them before I got them I have no idea .
My 1954 crosley has been running about 4 hours a day for a year and a half with no sign of any loss of brightness , it has a 17HP4 in it .which looks to be the original
I have used one tube tv for over 8 years , more or less 4 hours a day on a used replacement CRT that I got from another set . This set was made in 1974 . Perhaps there were improvment to CRTs by this time, it has gotten no dimmer with this use .
this has a 12VAEP4 and it has the rapid on system so that the filiments are lit all the time .

What's your experiances with CRTs