Brining time is dependent on taste- the longer you brine (and the smaller your chunks), the saltier your fish will get.
I don't like mine too salty, so I do ten minutes or so. Be careful with this timing; fourteen or fifteen minutes is MUCH saltier.
Start your timer as you begin to put your fish chunks (carefully cradled in your hands) in the brine.
The fish float, so an upside-down pot lid on top does a good job of keeping the fish in the brine.