It depends on what you mean by a "retention" enema.
Some people use the term to mean any enema you retain for any length of time, whereas others only refer to enemas that you hold until they're reabsorbed as "retention" enemas.
If you're doing a coffee enema, for example, you usually want to hold that about 15 minutes and spend 5 mins on your back, 5 on your left side and 5 on your right side.
If you're doing a probiotic retention enema, though, the goal is to hold it until it's absorbed in your body. You can either lie on your back or have your rear in the air and your head down on the ground resting on your hands/arms. You can stay in either position for about 30 minutes or until you feel you can move without getting the urge to expel it. When I've done a probiotic enema, I usually go lie down on the sofa after 15 or 20 minutes and try to stay there for about an hour and most of it's usually absorbed by then. Of course, I usually only use about 1 cup for a probiotic enema.