+1. The length of the samples should be of each users preference. One user would prefer 10sec another user would maybe prefer 30 sec samples.
Also, another option in this would be to set a percentage to play, e.g 5%, 10%, 25%, 50% and so on.
I've been at this before, in the Q of setting playcounts in the style of Winamp, where one can set %-play and sec-play.