Wastewater retention time depends on the load of organics and the biological components present in the wetland system. I had experience with ecologically engineered treatment three different tanks for diverse biological functions. This was successfully operated with 24 h HRT for each tank. Domestic sewage and effluents from hydrogen fermentation used in this case. Below appended link redirects to this article.
you just take the sample of waste water and kept incubation with orbital shaker, every day u will check all the parameters (physico chemical parameters) and calculate the percentage decolorization. normally the waste water RT is five days not exceeding seven days it depending upon ur dye containing waste water. if u want to any further research regarding waste water treatment interact with me..gud luck..