Drain time refers to the total duration of a health drain system, where the player's health is constantly decreasing as they play the map. 546470
Beatmaps with very long drain times are typically referred to as marathons due to requiring endurance beyond the norm in order to be completed.
The beatmap info page on the website calculates drain time (labelled Drain length) based on only hit objects' start times, so if the last object of the map is a spinner, the drain time will be displayed incorrectly.