Metadata 7 = 1000 Millibuckets, which is what is generated every 500 ticks in this case.
The Metadata used around the Spring is 15, which is 2000 Millibuckets, to imply a pressure release when you pickaxe into the Area around the Spring.
So the Math is 1 Bucket every 500 ticks on average or 2 Liters per Tick total, per bedrock Fluid Spring Block.