If I'm understanding u rite then the soil should have Lil 2 no nutes left.. If ur leaves r yellow now then I think there good 2 b chopped when ur gettin 20% amber crystals.. I would normally say give them 2 weeks of plain water but sense u been giveing them plain water the whole time I'm thinking there pretty much flushed by now.. I would juss pic the plants when the crystals show ripeness..if u got a ppm meter I would test the runoff water & make sure the nutes in the soil r almost gone.. If the ppm's r low then there ready 2 chop when done if the ppm's r still high (over 500) then flush some more till ppm's r super low (under 200)
But I've never grown in soil without adding nutes so I could b wrong here..