Slippery Slopes

Maybe some of the aspiring lawyers among my readership can answer this question for me.  I'd imagine that, if someone left a stereo in a public place, and that stereo were blaring an annoying, incredibly loud series of alarm-sounds on repeat, after some amount of time I would be legally allowed to turn it off, take it, or destroy it.  My question is:  what if the "stereo" is also a "car"?  How long do I have to wait?