Has the weather been pretty hot where you are? (please complete your profile to tell us where you live?)
Hibiscus flowers only last one day at the best of times, but if it's really hot and dry, you should be watering thoroughly in the morning so that the plant will have lots of water through the day. The flower edges are the first things to show stress.
If this plant is in a pot, you might try moving it to a spot where it only gets morning sun for the hottest part of the summer. Or at least give it some shade through the middle of the day.
