I don't think there's any rule about that, and most probably various factors influence the length of time for which a set is available. From experience, I'd tend to say that most sets last around 2 years, but it's certainly not fixed in stone, so don't take my word for it.
Considering exclusives usually mean more benefit for LEGO, it would make sense for example that well-selling sets are produced for a longer time - if it sells, there's less incentive to stop producing it, apart from production issues (the number of set in production at a current time being limited, and the same is true for parts so if the set has specific parts maybe it'll be phased out faster).
In the past, LEGO has even reissued sets which were in high demand, the best example being the Maersk ship for which LEGO had said there would be no more of them because they hadn't any granulate left of that colour, until Maersk intervened and a reissue was possible.
However, there is now another help for that, as LEGO recently created a "retiring soon" page which should give an good indication of soon-to-retire sets. It may not always be accurate, it seems to be limited to the US at the moment, but if your set is on the list, it's probably a good idea not to wait too long. But if it's not on the list, it doesn't necessarily mean you can sleep on it too long either.
Of course, some sets land in the Sales & Deals page at the end of their life, and when they do, you better act real fast or risk having to pay more than list price (example: the modular Green Grocer, which had been heavily discounted before selling out in a matter of hours, if not minutes. Good luck finding a cheap one now.). It may actually end up that way with the "retiring soon" page as fans hurry to buy the sets there, and there are indeed a few which are now sold out.
So, in short, don't wait too long if you can avoid it. Just because LEGO has sent a retired set to a kid doesn't mean they'd do the same for you :-)