wut up homie...the quick answer is 4-7 days imo but i go by the leaves... when they pale up and start yellowing pretty good then they are ready to chop... you want to use a lot of water at first to kick the proccess in motion and because if you drag the flushing out too long (start too early & go too long) then it will hurt your yield.
Coco can be flushed very easily and quickly compared to other mediums but the flushing time can vary depending on the strain, the size of plant/container, and the amount of salts that have accumulated over the grow from things like: using tap water, not enough run-off, high heat, ect... When it comes to coco, the hardest things to flush out are the big numbered powdered fertilizer like Shooting Powder, MOAB, Big Bud, Cha-Ching ect... And there are a number of things that could be in your tap water that are not so great and can be taken up by cannabis.
I've even read about cannabis (it was likely hemp) being planted at toxic spill sites because it is one of the only fast growing plants that can absorb a wide variety of toxins/waste/ect., and it can obviously tolerate less then ideal growing conditions.
For a one time flush you would need to use 3X the volume of medium, so a 15L pot would need 45L of water... its likey that no more flushing would be needed using this method. You can get away with breaking it into 2 waterings but it will take a little longer and dont wait more then a day inbetween the waterings.....
-J