The gardens is made out of multiple smaller gardens including: Cherry tree gardens, Japanese Hill and Pond Garden, Cranford Rose Garden, Water Garden, Alice Recknagel Ireys Fragrance Garden and many more. So it is safe to say that you would need at least half of the day to go through everything and enjoy the beuty of the gardens.
Apart from the gardens there are also green houses and conservatories such as the Bonsai Museum, Aquatic House and Orchid Collection, Tropical Pavilion and many more.
Best time to visit will be spring and fall, they currently have a fall foliage going on so you might want to check that. Tickets can be bought on the spot or online through their website.