Rain Water Harvesting Pit

I need to know how to calculate the volume of rain water seeped into the ground for 6x6x6 ft RW pit, if rain lasted for 15 minutes. We make RWH pits in to sensitize our community.

So, I want to write on the motivational Sign Board: 15 mins Rain: 1000 liters Water seeped into the ground, 30 mins Rain - something like that.

Please help with your experience.