The weirdest things make me homesick these days, like this brilliant picture of lightning striking behind the "Bat" building in Nashville. It is like a punch in the stomach, seeing something so recognizable in the Nashville skyline, and knowing I can't see that building any old time I want to anymore (not that I ever looked at that building on purpose, I definitely took it, and everything else about Nashville, for granted). The pictures make me feel so close to home I can almost touch it, before it fades away like a puff of smoke in my memories.
I guess the answer to the question "how long before you realized you were really homesick after moving abroad?" is 4 years.