Hi Stan, I've seen IBC reed beds here in Ireland, but haven't got much experience using them for reed beds. Generally I like to design my reed bed systems large enough to filter all the usual gunk that makes its way down the drains - including a flush toilet, dish washer, washing machine, shower, sinks etc. So the size is a lot larger than a few IBCs. When you're careful with what you put down the drain, you can afford to go a lot smaller with your reed bed size.
Here's what I'd do if I were using IBC tanks:
Saw the IBC in half, so that you get two 500mm deep tanks out of each unit. Fill these with
pea gravel and use grey water piping (PVC, eco-disaster) or 1" water mains piping (PP, much friendlier) as a connector between the two. Plumb the tanks so that you have a finished water level 100mm below the top lip of each tank.
If you're using the first one as a vertical flow reed bed rather than a horizontal flow reed bed I'd use a www.ribbit.ie
splitter unit to get a good distribution of effluent across the top surface; or build a syphon dosing unit to do a similar job. Then the outlet can be from the base part of the IBC, with an open tap dropping to the next IBC.
In terms of the time taken to process the grey water, that will depend on the amount of water you use. Here in Ireland the typical water usage is 150 litres/p/d (officially anyway, it's actually a bit less), and a typical grey water reed bed would usually be about 20m2 to get the water good and clean prior to the percolation area into the soil.
Check out solviva.com and oasisdesign.net for more background. Or of
course wetlandsystems.ie/watertips.html, but I'm biased... :-)