howdy! long time no talk
the flowers usually last as long as they do with a pond, sometimes 5 days... you want to cut the flowers early in the morning, the earlier the better but already blooming ones are fine. if you get them while they are still closed for the night, their metabolism is slowed, so when you cut it while its still closed it has more energy. they keep open and blooming as long as its sunny.
i hear you can even drip white candle wax on an open flower to keep it from closing up if your determined to show someone who is comin by later what the flower looks like.