I would normally put 5-7 drops (not "dropperfuls") fill the rest of this eye cup (which came with other eye wash kit) with distilled water and it's bearable. I mean it does burn and it has to, in order to bring blood to your eyes. It's still difficult for me to keep my eye open so that the eye will be exposed to the liquid but I'm getting used to it little by little.
After the burn, you'll start to feel your eyes getting hot (not burning) because blood has been brought to the eyes. It feels good.