I'm in the camp that I think any major institution like Shedd would take at least a full day to properly enjoy, which for me means not having to rush, seeing everything, and having time to watch/go back to things I want to see in more detail/again. It also means having time to do any extra stuff such as bts or rides if desired. And having time to eat and peruse the gift shop.
I also have to imagine that I would spend a whole day at Monterey Bay.
I am interested to know how long people think Brookfield and Lincoln Park take.
I think Lincoln Park can be done in roughly four hours if you skip the farm. I went from 11-3 in December and rushed the bird house past enjoying it, and fully missed a few of the hoofed animals, but had lunch earlier and watched the seal and gorilla demonstrations, which of course took some time. I had an additional two hours on my next visit (though I arrived a bit later) and was able to see everything (except the farm) with time to spare for a few choice exhibits.
The Shedd Aquarium is extremely easy to rush through, but I recommend taking your time there, intentionally. You can glance in each tank in Amazon Rising or Wild Reef and move on quickly, but I think the narrative is critical to what makes those exhibits special, especially when there are quite a few simplified Amazon and reef tanks in the country. I think the signage is worth giving special attention to there, but it will make your visit longer. I really recommend taking the full day.
Some will say you can fit Lincoln Park and the Shedd into a day, but I personally don't recommend it unless it is your only chance/opportunity to see both facilities. Better than nothing for sure.
Brookfield has always been a full day for me and as a child we almost never completed it. As a kid, we usually skipped Be a Bird (based on a a false impression it had no live animals) Perching Bird House and Reptile House, all of which I regret and especially wish I could visit today. We stopped visiting both children's zoos for a long time when I was a teenager, partly due to the upcharges at both.
I was almost a little shaken a few months ago to actually finish the entire zoo in a day (save Indian Lake) because it had always seemed unattainable, and that included both children's zoos, underwater dolphin viewing, extra time to try to see the pangolin (We just missed it, turns out) and a revisit to Australia House on the way out. So if you do open to close it's definitely possible to see everything, although if you plan to do a dolphin show that might mess things up, particularly since it's a bit isolated a spot.