Anything over an hour and usually i'll have headphones in to help the time pass, for shorter runs theres no point- just concentrate on breathing, pacing etc. I never have it too loud that I can't hear road noise etc. Never listen to music on the bike it's an invitation to be crushed by a lorry or some other disaster.
I dont mind that you cant have music on during a race- it aids concentration if you're just whipping along with the noise of the tyres and the noise of your breathing- and the noise of the laboured gasps of the guy you've just dropped on a hill fading into the background.
