every 8 - 10 day...
depending on how fast the soil drys, every other "watering" is supplemented with a 1/2 to full strength feeding.
like i said, i was feeding them almost every watering, but i don't have an RO filter, or the ambition to buy or distill water. So.... i would have to PH balance my feedings and pH-up them, causing the aforementioned K def.