Hi Propaul, those enema retention times you listed sound reasonable, and are probably good guidelines for people looking for looking for recommendations on how long to retain an enema in order to achieve best results.
When I was growing up and my mom would give us enemas she always prepared them by swishing a bar of Ivory soap in a pitcher of warm water. After we had taken the required amount she had a little 3 minute egg timer, the type like an hourglass with sand that takes 3 minutes to empty. If we weren't very constipated we would only have to hold it for the time it took for the sand to empty once.
Even though we could watch the sand dropping, it seemed like time was standing still. On some occasions we had to endure 2 or even 3 turns of the little hourglass. That was the worst part of the entire enema!