I use a slot washer. At first, I have water moving quite fast, maximum speed without overflowing which isn't actually much. Then I slow the water movement to bare minimum. Then wash for 30 minutes. Rationale is, I want to get rid of what's on surface quickly. Rest is diffusion anyway so it's going to be slow. I just want water moving.
So far, no issues. I have no idea what the total water usage is, but it can't all that much looking at how much it flowed out.
This reply doesn't establish anything in terms of minimum but this is what I do.