The fountains put on their show -- wonderfully timed to music choreography -- every 15 minutes or so after dark and last for a few minutes. It's very pretty and lasts for the duration of the song. Stay for many other songs, especially the Michael Jackson one. The water streams move in the Jackson way. Don't go on high wind day, not only you get wet, the show may be stopped for that reason. There is plenty to see at the Bellagio, so time your visit so that you can pick up at least two fountain displays.  Also: be sure to visit the "conservatory" inside the hotel for beautiful floral displays, seasonally  changed.