How long can preamp tubes last?

Some tubes can  last for years.

Rock, trust me on this. You will absolutely know when a tube goes bad.
If the preamp sounds good. Leave it alone. It never hurts to keep a spare set of tubes for when the tube, tubes do go bad. Do yourself a favor. Do not get started tube rolling. It will drive you crazy.

Pre amp tubes last a long time, many years, and i would recommend nos tubes from someone like andy's tubes, and tube rolling means switching out tubes for different ones, whether old or new. A good nos tube is expensive but well worth the extra cost for any unit.

450 hours a year? My cables take longer than that to break in (so the manufacturer says). I've had the same NOS GEs in my Freya for a long time (2 billion hours or so), and the interesting and somewhat unique thing about the Freya is that you can check the honesty or whatever of the tubes instantly...simply switch the mode to passive or FET and match the gain...presto el checko. If the tubes (relatively) sound like mush possibly change them, or for you, get new ones anyway in 2036.

Tubes in my Mcintosh C220 have been in place for almost ten years, used on average about 15-20 hours per week.  When I had the preamp serviced this past fall, I had the tubes tested.  They tested like new.  So, the answer is, a very long time.