We spent a solid 8 hours and could ha e easily spent another 3-4 more. But we had a mixture of people who enjoy art, history, literature, plant, and walking. If you're only interested in one of those things or only passingly interested in a few of them then it could comfortably be a 2-4 hour trip.
This is really dependent on you and your group. Some people are good with a couple hours, some make a whole day of it. There is a lot to see here, so pace yourself but make sure you take a look at the map to figure out everything there is at The Huntington.
About 3-4 hours is good or more as there are many areas to check out within
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