For any word that you always get correct, this is the review schedule:
Plant/learn: review again in 4 hours
First review: water/review again in 24 hours
180 days, etc
If you get a word wrong during the planting session, it will come due in 4 hours, and then 12 hours. After that, it follows the normal schedule if you always get it right. (24 hours, 6 days, and so on.)
In some courses, if you plant, and then don’t water for 24 hours or more, Memrise adds an extra watering session. (4 hours, 12 hours, 24 hours, 6 days, etc.)
Later, after you’ve watered an item a few times, if you get it mostly right (maybe you had a typo), it will come up for review in 4 hours, and then return to the normal schedule. (For example: plant, 4 hours, 24 hours, 6 days, got it mostly right, 4 hours, 12 days, etc)
If you get it completely wrong during a watering session, the schedule starts over as if you’re planting. I’m not sure if it includes the extra watering at 12 hours or not.