Wherever you heard that its major BS. Theres no set amount for how much water you use to flush, the last thing you want to do is drown your buds, or get bud rot or root rot.
Its not how much, its how long. Feed them the same amount of water as you were when you were feeding with nutes, just use plain water, or if you want you can use a flushing solution (final faze or somthing)
How long to flush is really up to you, some people flush for 2 weeks, some people for one. I usually flush for just over a week then let them dry out a little longer then normal before cropping.
It really depends on the strain and your grow room.