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

you don't want to leave tube gear on 24-7

yes i think some of your tubes, based on your description, can stand replacement - when you get new ones, see if the sound improves, then you will know

Yeah, no need to leave on 24/7. 
Replace after 5000 hours.

Thanks for the replies jjss49 and noromance.  Sounds like I'm well over.  The PPT stuff made such a difference in performance, I didn't notice the drop-off in SQ until this past week.

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 ...

https://www.ebay.com/itm/Accurate-Instrument-Tube-Tester-Model-157/154119691696?hash=item23e23fddb0:...

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.

https://www.ebay.com/itm/Accurate-Instrument-Company-New-York-NY-Tube-Tester-Model-257-Vintage-WORKS...