Super Mario Odyssey – Features of the Skeleton Suit

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)

As of right now there is no way to do this. I've beaten the game, each world in it, and completed several (some coins, some moons, some both). There does not seem to be any hidden options or NPCs that would trigger something like this.

The only thing that fits this criteria is replaying the Festival in New Donk City, but that's only allowed because there are moons hidden in it, so this prevents you from permanently missing them.