Answer:
I've actually found myself asking this too! The answer I've found is any length of time, as long as you can explain why it's significant to you. Not just (for example) I'm helping the animals at an animal shelter, but maybe- it's taught you to take on more responsibility, or work collaboratively. However, maybe less than a week isn't great? I would say a few weeks at least, to show commitment as well. If you really think that volunteering at xyz for 3 days so far has really contributed to your character, then go ahead and put it- just make sure you have a good explanation as well.
I hope this helped!