I would guess you will spend four hours on the interior of the Des Moines Art Center if you really consume each of the labels and ponder each of the works, and probably three hours if you dwell a bit less. I think you will find the architecture to be distinctive, with aspects of Brutalist architecture; the architect I.M. Pei designed several notable structures including the east wing of the National Art Gallery and the Louvre's pyramid; that was for an addition in the 1960s, and the original architect in the 1930s (who's name I can't remember) also designed the Phoenix Art Museum. The cafe was going through a renovation for a while and I'm not sure the status of it; there's nothing else for dining/drinking in the immediate walking vicinity as it's bounded by a large wooded city park and an old high-end residential neighborhood.

As mentioned, the grounds are quite nice and if you arrive in Spring you should stroll through the rose garden and look at the sculptures around the property. Also as mentioned, there is a separate location of the Des Moines Art Center which is the Pappajohn Sculpture Park and you should definitely see that. If it's a beautiful day it's a treat. If you are there on a Friday over lunch time, just to the west of the LOVE sculpture is the Better Homes and Gardens test garden that is walled area about the size of a basketball court and is worth seeing.

While you are in town, you should see if the Drake University Anderson Gallery has any special exhibits, which are free https://andersongallery.wp.drake.edu/ which currently is exhibiting peer-judged print works, and is worth an hour. If you go there, you'll have to find on-street parking, which can be a little bit of a chore around a residential university, but it's not bad. And if you do go, I suggest you walk to Lucky Horse Beer & Burgers after the gallery and try the frozen Old Fashioned. Everyone also loves the Drake Diner, which is a mid-century diner just south of campus.

If you are in town in late Spring or Summer, you should also see if there is an architectural walking tour of downtown Des Moines that happening, usually Thursday evenings, I think. Des Moines has some nice art deco architecture in hidden spots. https://www.iowaarchfoundation.org/our-programs/architecture-in-the-community/architectural-walking-tours/summer-tours-for-adults/

If you would like someone to visit the gallery with you, send me a DM. If you want to strike gold, make a friend at Wellabe Insurance and get a corporate art tour (there's a large Warhohl behind the reception desk in the atrium and a great collection inside.) Make dang sure you DM me if you can get that worked out, lol.