How long will tubes last in a pre-amp?

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.