Keep it sharp and keep it out of your puddle,  the longer you you keep it clean the longer you will have it.  Never weld with a dirty tunksten.

I sharpen my tungsten every November, whether it needs it or not..  :)

Depends on the current level you use.  Higher current levels errode the tip more quickly.  Alloy matters too.  Also depends on arc starting method.  Scratch is awful tuff on the electrode compared to HF or Lift.

I think a good welder using a 2% Th on DCEN at a moderate current level and using a well designed "lift arc" could probably weld out a 8" pipe Sch 40 from start to finish without grinding the electrode if they had to, maybe even a Sch 80... althougth I like a fresh and perfectly dressed end for the cover passes myself.

yah electrode life is usually a function of current density and thermal cycling, A.C welding of aluminum is much harder on electrode life than say steel or stainless steel.  the other big thing is if the shielding gas covers the electrode 100% until it's cooled. IF the post purge is too short the electrodes tend to oxidize

In automated welding we would usually switch out electrodes about every hour of arc-on time.  Too many start-stop cycles can cause the sintered electrode tip to degrade and fall in the weld pool.  So there's another limit.

IF you had an automated orbital welder you could probably get a few hours of arc-on time out of the electrode before it had to be redressed, refractory alloys will condense onto electrodes and degrade them over time, especially at small arc gaps.