Whoever wrote the texts in the New Testament was clever enough to specify it vaguely. Short, for god, can be very long for you, mere mortal. Of course, there slipped an error in some of those texts where Jesus promised that this "short" means "within the lifetime" of those who he was speaking to. But, again, the Bible is so full of much more blatant inconsistencies that this got drowned down below. The believers don't mind, though...