How much time do you have to say “Hello” to another at a gathering before you should be saying goodbye? I guess it all depends on the size of such a gathering and when people are starting to say goodbye. It also might depend on when you arrive at such a gathering because nobody expects your greeting until you arrive. If you start with “Hello” to late it might be the only one word or expression of affection one can express before the door is closed. Hello. therefore is not a fixed in time although time is a moving before goodbye’s are in order.