As I've mentioned to Carl before, I was trained by Dr. Tharp himself (The original researcher for development for standardized field testing.) in the early-mid 80's on what is now a standardized test. It was a three to five day course course for Drug Alcohol Recognition Training. (DART) But, the field test was the real focus as it was his baby.
While the documentation and reasoning for the tests took up much of the time (along with the focus on consistency), anyone who couldn't learn the FST battery in an hour is an idiot.
Clearly, more experienced officers are usually better. But then, they have numerous chances to take their observations and check it against the breathalyzer. (For us it was Intoxilyzer sumthin' sumthin') As in anything, if the officer sincerely tries to do the test and then compares his thoughts with the eventual results, he will get better.
But, knowing how to conduct FST is not something reserved for brain scientists. I suspect the number of hours needed for training is not based on the difficulty of the subject. (At least as to the FST for alcohol. I have no direct knowledge of the whole range of options available to what is now called a DRE.)