According to the "official" tourist guide for Beijing zoo:
Beijing Zoo
Recommended Time for a Visit: One hour
One hour? Not a chance! This place is pretty big - I think you'd have to walk pretty fast and only glance at each exhibit to get around it in an hour.
If you add the aquarium to the visit, there's another hour gone at least - especially if you watch one of their shows.
It just made me laugh when I read that
Beijing Zoo
Recommended Time for a Visit: One hour
One hour? Not a chance! This place is pretty big - I think you'd have to walk pretty fast and only glance at each exhibit to get around it in an hour.
If you add the aquarium to the visit, there's another hour gone at least - especially if you watch one of their shows.
It just made me laugh when I read that