Yes, one day, thus the name Daylily (genus is Hemerocallis so not related to the Asiatic or Oriental Liliums)
From the Daylily Society site:
"The word Hemerocallis is derived from two Greek words meaning "beauty" and "day," referring to the fact that each flower lasts only one day. To make up for this, there are many flower buds on each daylily flower stalk, and many stalks in each clump of plants, so, the flowering period of a clump is usually several weeks long. And, many cultivars have more than one flowering period."
