I experimented when I brewed last week. I know this is all qualitative..., in previous batches my 20ft 3/8 in. IC at a slow flow rate for a 5 gal batch would cool from boiling to 100 in 45 minutes using 10 gal of water. I increased the flow to use just over 15 gal and it decreased the cooling time to around 15 minutes. I move the IC up and down constantly while running the water. Looks like my garden gets a little extra water on brew days.
I am still planning to get a hydra IC sometime this winter... So I can brew and cool larger batches.