Walking leisurely around the garden would take about an hour. Longer if you really want to learn about the plants and spices there.
There is a cooking school where you can learn how to use spices in your food. So if cooking is your thing, it's worth spending some money for cooking lessons. Advanced booking is required.
The gift shop was amazing. There are various types of spices for you to buy. I hoarded some essential oils and soaps. After I paid, I was told that I can also buy their product online. Duh, If I knew I wouldn't have bought so much that day!