The first instructor I had told me I needed 20-30 hours of lessons to be ready for the test, which rather surprised (and upset) me. I'd learned on a manual when I was 17, had been driving automatics since then (I'm in my early 30s), and had been driving a manual daily in the UK for about 4 months before I started my lessons.
I switched after only a lesson or two with the first guy, and had two lessons with a new instructor before he told me I was ready, to go ahead and book the test. I passed without any difficulty.
I always recommend now that people switch (and keep switching) 'till you find someone who helps boost your confidence, not tear it down. It's no good going into the test petrified!