I've always assumed it did not happen that long ago... Maybe a few a days earlier, or maybe just yesterday. For it seems that it is typically thought that the characters start out of the blue in the situation, maybe because they were just then washed on shore or something like that. It is not very plausible that they were on the sea for years or even months without knowing about this z-calypse. The time-line is bit muddy for many reasons. For example, as the "zombies" are presumably not undead or supernatural, but rather living "infected" humans with human physical capabilities, they should not ipso facto be able go on without eating and drinking for a very long time... actually they should propalby all be dead within couple of months at most. (Especially as it seems they aren't very keenly seeking water to drink...) And why is everything so worn and rusty? Maybe its just the Chernarus thing... not much bothering with crops, leaving cars to rust, never ever repairing a fence etc... But anyways, I suppose it hasn't been that many days since z-apocalypse.