How long should the retention be? Well it depend on the solution, the temperature and the volume.
I cannot retain a 2 quart soapsuds enema more than 3 or 4 minutes, a 2 quart saline solution over 10 minutes, an acidic (acv or lemon juice) enema over 1 minute. However I have retained a 2 quart saline solution after my normal BM for over 15 minutes. 2 cup coffee enemas I retain for 20 minutes. A one quart coffee enema I usually try to retain for 20 minutes. I hope that this gives you an idea of what others do.