The lifespan for a grind to last depends how harsh you grind it whether its light,medium or hard and also depending on how often you ride too... I tend to grind mine every 3-4 months or when my brake is going pants or the grind fading. I have grinded my try-all rim atleast 4-5 times now as before i use to go smooth rim and always did medium grinds and to be honest theres still loads of life in the rim to do plenty more... Plus its always best to grind with your tyre on as every time you take your tyre on or off you are contaminating the rim which can effect your braking performance.
Thanks Andy