How long will tubes last in a pre-amp?

My Sonic frontiers Line One seems fine, tho I haven't put anything else in place of it to test.

It is a decade or more old.  Should I get worried about the tubes?
Preamp tubes are not too stressed, they can last a very long time, except in this one I know of, Audible Illusions 3A they ran them very hard.
Check for micro-phonics though by gently flicking the top of each tube with the finger nail while at normal listening level, have some one listen at the speakers, very small noise is normal, loud ones can mean this tube has gone miro-phonic.
In my ARC REF 3SE I replaced all the small tubes by 6H30P-DR ,
they should last more than 10.000 hrs.
Why not replace them? Preamp tubes are inexpensive. Ten years of use with any tube will wear down!
Every pre- amp is different usually 5 to 10 thousand hours but it depends on the voltage they run at.
What does "wear them down" mean?
I am still considering the CJ classic 2 SE Preamp

just not sure about having a tube. S is so much easier. 

Randy...look up cathode depletion. 
I'm aware of cathode depletion, but by that time, the tube would usually fail completely, not just show degraded SQ, AFAIK.
Preamp tubes for the average use listener should last 10 to 20 years
Left a 12ax7 on 24/7 for 10 years and it still tested good
Google search "Sonic Frontiers Line 1 tube rolling" and see what you come up with.  You can really change the sound of that preamp with different tube types.  I prefer Amperex Orange Globe 6922 with A-frame getter.  Great sounding tube for the Line 1.  Good new production tubes would be Gold Lion IMO.  The tubes you have in there are probably fine Randy, but tube rolling is good fun!
Probably like many things, as soon as you stop worrying about them, *poof*.  Buy a set of back-ups and put them someplace safe.
Long before the tube fails completely, it can become progressively weaker.  Depending on the design of the linestage, it is possible for the unit to sound reasonably good with weak tubes in it or it can become quite lifeless sounding and noisy.  There are VAST differences in how hard any particular unit runs tubes, so one cannot merely count the hours of usage.  There were, for example, Counerpoint linestages that would run through a set of tubes in less than a year's time if used every day.  The best way to deal with aging tubes is to have multiple extra sets of replacement tubes.  Once in a while you should swap in the new, or less used, tubes to see if the sound of the tubes in use have degraded enough for you to want to replace them.
Ask Chris, he'll tell you straight up.
I have yet to see a preamp tube fail, in contrast I've had a few failures in power tubes (KT120, KT88) that were either silent or resulted in a sudden burst of illumination and no more music...

SF Line 1 uses full filament power but a very low B+ so the tubes will be running without cathode stripping.  It is designed to have the tubes warm without being at the expense of tube longevity.  
I ran the tubes in my  CJ PV8 for over a decade with no problem or noise.  However, when I upgraded them, WOW, it was like a whole new amp, a huge improvement.   As the poster above, noted, you might rolling some different tubes through the unit, too.   Different tubes can make subtle to profound improvements in sound.