I have never heard of that long of a break between trims. If your horse is running in pasture with lots of natural hoof-wearing terrain and has a tendency to chip/wear perfectly evenly, I suppose you could get away with that. I'm in the US so don't know much about NZ geography. However, we're in winter up here and you're in your summer in NZ, right? I could get by until March, in a pinch, but not till May....but hooves don't grow as long in the winter as in the summer.
You say he's beginning to flare so that tells me that he's not growing out a perfectly healthy, well-shaped hoof just from natural wear. If you like her and how your horse's feet look after the trims, and want to keep her, ask her to explain. If she wants to keep you as a client she'll respect that.