Any process that results in a change in state or activity of an organism (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of detection of, or exposure to, a day length that exceeds a particular duration known as the 'critical day length'. The critical day length varies between species. Although the term long-day is used, most species actually respond to the duration of the night, so that the response will occur when a period of darkness falls short of the number of hours defined by 24 hours minus the critical day length.