How long is it before a plays ending is changed by the audience

Your sitting in the Theatre watching a play – just before the interval – the actors stop, come out of character and tell you to get your phones out and to vote using Twitter on the way that ┬áthe play should end. This might seem really far fetched but it isn’t at all, already within Live TV performances such as the Xfactor, the shows producers are watching the twitter stream and changing elements within the show as it goes out...