Someone you're in pretty regular contact with, perhaps monthly "how are you" texts and the odd meme etc. See each other a couple or a few times per year, but then all contact stops even though you've sent a couple of breezy messages. No fall out at all, just leading different and busy lives.
At what point, if you had to put a pin in it, would you write them off, assume they've moved on and just obviously aren't bothered. I'd say around one year.