It's been my experience when fasting than an enema helps me feel better in a variety of situations. When tempted to eat, when depressed and tempted to give up and quit, and sometimes just to experience the healthy, clean, empty feeling I usually experience after having an enema.
There is no best time to take an enema other than when you feel it would help you feel better. I've taken them first thing upon waking in the morning, in the middle of the day and at bedtime. Whatever works for you is the right time.