I try to just make enough so I'll use most of it. It starts to breakdown into various compounds and will begin to tear apart your plants tissues. You can also try mixing in about 1 TSP of plain, unscented dawn dish soap. It helps the neem oil molecules emulsify better with the water. When I am on a neem regimen, I'l rotate one day neem/one day fresh water to remove residue. You don' want that shit building up over leaf stomata. Will fuck your shit up and make it hard for plant to respire.