I filled up a big bowl of water, put in the castle (I have a sand colored resin castle), dumped in the bleach, and let it soak for a couple hours. I never measures how much I put in. But afterwards I boiled the castle for about an hour, then soaked it overnight in 4x dechlorinated water then put it back in the tank. It was spotless and didn't have any bleach on it.
Do you have the castle that is in my gallery?