How long can I expect tubes to last on a preamp or MW CD player?

Secondly, what would be the signs of tube deterioration?  Without a tube tester, is there any way to tell how much life they have left short of they aren't working anymore?  I have 7 tubes between the TRL DUDE and MW 5400 Signature Truth.  I may have hit that point on at least a couple tubes.  Seems to have dropped a little in SQ.  The 2 SNL7 tubes on the MW are only a couple months old.  The rest have been on 24/7 for at least half of their lifespan, about 4-5 years with NOS tubes in them.  I still have the original tubes with very little use.

dorkwad is a good name for anyone who leaves their tubes on 24/7.

typically, in use, 10,000 hours.

I have one of these simple testers, love it, lightweight, portable, take it to my friends houses ...

I check em all periodically, and if I suspect something, one side weaker ... more often than not it ain’t the tube, a loose cable .... I collect old tubes, people gave me many years ago, mostly used, flea markets, their dad dies, out of the basement comes boxes and bags ... ...

check em out later when I or someone needs one, they are nearly all good, the most common problem I find is shorts which that little tester shows easily.

Also use it to check my new tubes and to check matched pairs or quads,

it always agrees with my big bulky hickock tester,

this one, a later model has the instruction book.

Thanks for the compliment--I resemble that remark.  It will change my life.  The rest of the comments were interesting reading and maybe helpful.
I apologies for falling prey to too easy a trap for sarcasm.

let's get back to tubes life 24/7. really? what/where/who/how could you think they need to be left on? or did you just mean you listen a lot every day?

new tubes, break in: leave them on, no signal needed, go to bed, 60 hours. off. on, warm up 10 minutes, now that is how they are gonna sound. off when not listening.

as they age, still equal l/r? have favorite music that helps your awareness of strong center imaging, equal l/r. cd version best for this, just in case cartridge is not perfectly setup. actually, have both lp and cd versions, play cd, all good, play lp, still good? great!

this is my favorite test for balance/imaging

side 2, tracks 2,3 all 3 guitars, and equal audience sounds l/r

I just re-read your OP.

You can switch l/r, any volume balance change?

You can put the original tubes back in, different volume? if obvious, get some new ones.