Well there seem to be some fairly well defined limits in amongst the replies so thank you.

The more I think about it the moreI realise that there are huge problems in setting up an experiment to produce a sound that actaully is the length that you want and avoiding complications of prolonging the sound with echoes. You would have to use a high quality in ear earphone I presume and even then you would have to check that it didn't have a decay vibration after the note stopped.