How long should I allow to be here? Google's "people typically spend" estimate is usually slightly off, so what, around maybe 3 hours or...?

3 hours would allow a casual stroll through entire collection, indoor and outdoor with adequate time to stop and note/admire particular installations. Dont forget to explore the outside beyond the garden area... people miss walking around building and discovering the quietest "hidden" areas and artworks.

3 hours is plenty of time to go through the indoor museum and the out door garden. There are several nice pieces inside so really it depends how long you stand and stare at them. If you are with kids one and half inside and 45 minutes outside garden tops..

That is plenty of time

3 hours would be fine!