Lilies and other aquatic plants begin to go dormant about the first of September. If they stop blooming in the summer check fertilization schedules and amounts. If the leaves are getting smaller they are not getting enough fertilizer.
A tropical water lily bloom lasts about three days until a new one takes its place. Pinch off the old flower and discard it. Do not let it rot in the pond.
Your lily will be happiest if the top of the pot is at least 6" below the surface of the water....deeper if you can. I place them on the bottom of the pond (18").
Water lilies like to be fed every 10 - 14 days with Pondtabbs or other aquatic plant fertilizer. They also like to have the tops of their pots at least 6" below the surface of the water and are happier even deeper. And remember, the outer ring of leaves gets yellow and needs to be pinched off periodically so the new leaves can grow. Same with the flowers.
If water lily leaves begin to disappear, check your pond for visiting wildlife.