How long after taking enema the bowel empties out? should one drink several glasses of water before taking enema?

The amount of time it takes to have a bowel movement after taking an enema varies, depending on the amount of stool present, its consistency, etc. Typically, it works within a few hours but this can vary if the stool is very hard, there is a lot of it, etc. It does help to be well hydrated and to maintain hydration after the bowel movement. If constipation is a big problem for you, see a GI doctor