Super Mario Odyssey – Cascade Kingdom Floating Island Power Moon

It varies depending on the plant’s size. 3 leaves grow every 3 minutes, while the bulb takes 9 minutes to grow.

Based on my experiments in the Sand Kingdom, I initially assumed that each plant took 21 minutes to grow to full size. However, I have since learned that the time it takes will vary depending on the size of the plant.

On average each plant should take around 15-24 minutes to grow, but it can vary.

A few notes:

• The largest plant was in Seaside Hill. It had 45 leaves and took 54 minutes to grow
• You can make the plants in Seaside Hill grow faster by capturing a squid and watering them. (Thanks raznagul)

Old answer

Based on Nolonar's comment, I knew that the plant would grow if I left my Switch idle. So I went to the Sand Kingdom, set a timer on my phone, and waited.

0 - 12 minutes

For the first twelve minutes the plant gained three new leaves every three minutes. This means you could use leaves to tell how long the plant had been growing. (3 leaves = 3 minutes)

12- 21 minutes

No more leaves grew after the 12 minute mark. For minutes 15, 18, and 21, the yellow bulb at the top grew in size. After 21 minutes the plant was fully grown.

Here's a screenshot gallery showing the plant growth. (minor spoilers for post-game outfit)

Yes, there is an invisible wall at that location.

A recent video posted on Reddit clearly shows Mario wall sliding, then diving into an invisible wall above the building in question.

Also, as Fredy31 suggested in the comments, I have returned to the area (after completing the main story) and Cappy confirmed that there is nothing else to find in the area, so there can be no Power Moons or Purple Coins above that building.