To be honest, you could carry on washing it for days - like many substrates. I just washed it until it ran clear, then put it in the bucket. After planting up I filled the tank carefully as usual, was only slightly cloudy and it cleared in next to no time. I was using mostly mature water though, as I only changed the substrate .
I found that the water became crystal clear far quicker with Tesco Lightweight Cat litter than any other substrate I've used. Even when setting up from scatch it was crystal by the next day.
If you have a problem with it clearing, add some mature tank water and fliter over filter floss for a day.
By the way, the three tanks have now been working for over a year and are great, semi-Walstad style, I haven't changed the water for 6 months and both fish and inverts are fine, plants are fine. Water is always crystal clear.