A lot less than you think!!! See the empirical tests in this article.

http://www.wetwebmedia.com/BulkheadFloRateArt.htm

Main take away: the standard Megaflow 1" Durso drain will only provide at best 380 GPH of flow, *not* the 600 GPH advertised.

If you think you have more flow than that, you might want to actually measure your throughput. Your pump may not be putting out what you think it is.

I've been trying to figure out why my 22" ATS running off my drains seems like it isn't getting enough flow, even though my drains are close to maxed out. It turns out that running at their best rate, the dual 1" Durso drains on my AGA 125 gallon combined will just barely give 35 GPH/inch. In reality, I image that they are running substantially less than max. This also means that my Mag 12 return pump is putting out a lot less flow than I thought. I did have to dial it back when the system was new, but over time I've opened it up. I need to take some time this weekend to clean the pump and actually measure the flow through the system.