Google Search, not Google Maps, is adding a new feature when you search for a place. You'll get the usual card with the photo, location, open hours, description, reviews, and more, but there will be a new section there called Plan your trip with the duration people typically spend at this location.

This information won't show up for every single place you might look at. It will most likely be limited to popular touristic and historical locations with lots of foot traffic where Google can aggregate data from different visits and figure out how long people usually stay there.

This, along with the Popular times graph that was added last year to Maps (and Google Places) should help you plan your visits to different destinations more effectively, especially when you're on holiday or traveling.