You learn it, but academically. Growing up with everything in feet, miles, pounds, etc... you get a feel for whether to put on a sweater if it's supposed to be 60 degrees out or not, or how long it might take to drive 10 miles on the freeway, or about how far 20 feet is. So even for someone like myself who has lived in Europe for a long time, metric units don't feel quite as 'native' or ingrained, except for temperatures, because you deal with those every day.