A dj mix should last for ages really, but it would take ages to listen to everyone's mixes if they go on for over an hour each time! However, making it a mere 15 minutes should seperate the wheat from the chaff as it will take some madcap skills to wedge a decent selection of sounds into such a short timeframe (but as i generally play more tunes in ten minutes than the average organic jukebox plays in an entire set it shouldn't be a problem!).