It depends on how clear your colon is. Sometimes it's just a few minutes for me to do an initial cleansing enema before I cah then retain one, other times I have to work through pockets, so it can take some time.
There are so many different methods, but just a google search on "how to do a high enema" will give you some good ideas, that's what helped me to understand.
Putting a little salt in the water helped me so much, it keeps the water from being absorbed. You then want to make sure the bag is rinsed well before doing a retention one of course.