There are other fields of endeavour somewhere between 'rocket science' your 'everyday life' as an American teen. There are designers, architects, engineers, builders, cabinet makers, manufacturers, chefs, surveyors, and many others who, believe it or not, do need to be fairly precise with their measurements. And sometimes, one needs to do calculations with those measurements.
To answer your question, when somebody asks me how big something is, it really depends on what they're referring to. Depending on the context, my answer may be any of the following:
a. About 40 centimetres
b. Let me grab a tape measure
c. None of your business!
It's never been much of an issue to me. Somehow us dumb Americans have survived over two centuries of designing and building very complicated structures and assemblies using our inferior measurement system.
Oh, by the way: Engineers often work in decimal inches. Makes working in inches pretty easy.
IMO it's a non issue. Maybe we'll convert to metric when the rest of the world starts speaking English.