How often should you resurface a gunite pool?

That depends on meny things.

If the pool has been maintained throughout it's history under pristine conditions you may get upwards to a 40 yr. lifespan. That means above all proper water chemistry always in balance water. But that is an unrealistic thought with most homeowner/pool persons. If pH and Total Alkalinity are not maintained within recommended scales then you cannot expect more than a few years life out of the plaster surface. That could range anywhere from 6 months to a year with extremely low pH levels. Water gets aggressive at the fringe of the pH scale. At the upper end - pH readings consistently at, say, 8.0 you get scale deposits. These will also affect the heater and other equipment. At the other end of the spectrum, readings below 6.8, even for short periods, you get etching. This also has a decremental effect on your heater, pump and other equip. Either situation shortens the life of pool and related equipment. k