Plant lovers will love it, simpletons not
I can't imagine anyone would merely take 30 minutes to see the gardens. If you ran through the greenhouses alone it would be 30 minutes and not really looking at a single tropical plant or sniff an orchid. Outdoors the two gardens are a nice walk; one has specialty gardens, the other a woodland tract that is magnificent in April/May when wildflowers and rhododendrons are in bloom.
Plus, there is NO charge for access, and no parking fee if you follow appropriate visitor parking tag procedures right there at the parking services building. You can buy plants there if you want.
Plus, outside of the North Carolina Botanical Garden in Chapel Hill, this is really the only place to purchase a variety of native and hybrids of pitcher plant, venus flytrap and other bog plants.
If you planning on seeing plants jump up and down and sing for you or take a dump on the lawn like what happens at a zoo, this place isn't for you. It's relaxing, interesting and a way to take a nice casual walk and encounter various plants (indoors and out), garden rooms and sights of water features.