Electrode depletion is a possible EOL. Another failure is if the glass walls absorb too much argon, which prevents it from striking. Other times it could start "distilling", where some places of the discharge tube lose sodium and therefore get hotter, which repels more sodium away. Or they could start to leak sodium into the outer envelope. But yeah electrode depletion is probably the most common as far as I am aware.
Most SOX lamps are rated for 18,000 hours, so supposing you run it for 12 hours a day it would last over four years. I assume you are running it less than that, so probably a lot longer than 4 years. Though many different factors play into the lamp's actual lifespan, so who knows. I am sure someone else can chime in on the specifics of that.