It really depends on the audio file you're trying to use. Some sound fine within 20 seconds, some sound fine with 40. Really can't generalize.Yeah, but for best usability 30s is the goal.
Also I hope other viewers can see this and able to make decision themselves.
+1 on others making the decision themselves. Another pro tip - if you're trying to use an audio file on a mac, you can use QuickTime to trim and export it.