The Guggenheim stays open to 7:00pm most days and most people only visit for about an hour. As others have said, the architecture is the big draw for the Guggenheim and the collection is secondary. while It looks huge from the outside, the actual area available to exhibit art isn’t that large, especially if you subtract the space used for Richard Serra’s monumental “Matter of Time.”
i would suggest you visit their website to get a feel for their collection and see if it really interests you. They have a decent range of artists in the collection, but the only works that stood out on my visit were El Anatsu’s “Rising Sea, Jenny Holzer’s “Installation for Bilbao and Serra’s “Matter of Time,” the rest of the works inside were rather average works for the artists.
Their website will also inform you of what exhibitions they will have in June. On June 27th they open a new Yayoi Kusama exhibition that should be interesting, but it no doubt swell the crowd for the first couple of weeks If not longer.
If you are going to the Guggenheim because you really enjoy art as opposed to going because you have heard it is a must see, then you should have a good visit. In that case I would also recommend visiting the nearby Bellas Artes de Bilbao.
Three pieces of art that can be viewed from the outside along the riverside that most visitors miss are Yves Klein’s “Fire Fountain” which operates from about 19:30 pm to 20:30pm, Fujiko Nakaya’s “Fog Sculpture, #08025 (F.O.G.) which operates on the hour from 10:00 to 20:00 and Jenny Holzer’s “Like Beauty in Flames” which requires downloading an app and a good wireless connection.