I personally had 3-5 years of symptoms related to B12 deficiency, and my level was 294 (150-1100) when it was discovered. Thankfully, the neurologist who thought to test for it knew that low normal levels could cause problems in some people. He was the eighth specialist I had seen for my symptoms over three years, and the first to think of it.

Check this out for plenty of references and a list of the symptoms I personally had with a B12 deficiency: