What was it? Rods to the hogshead?
I remember an article in Scientific American (or something like that) a couple of years ago, where someone suggested that American students, on average, lose an entire semester doing conversions and learning the EEU/SAE (or whatever you feel comfortable calling it) system. Now, that's unlikely, but a month or so is fully possible, in my opinion.
Edit:
EEU = English Engineering Units
Probably the best "name" for that unit system, just as "SI" is for what's normally referred to as "metric".