Short answer: when I'm horizontal and starting to absorb the good stuff.I think though that, in addition to holding time, absorption qualities of the rectum and colon lining are very important for a successful coffee detox.
Waiting an extra day after having used a greasy or silicone based lube, then letting a high fiber diet sweep things clean might be just as important as an extra 15 minutes of enema retention.Since the objective of the coffee enema is to keep most of the solution in the lower bowel where there is maximum exposure of the Portal Vein, -- http://en.wikipedia.org/wiki/Hepatic_portal_vein -- , I tend to like the old Balch procedure which divides the coffee solution into two small 24 oz enemas. I take each standing up in my shower (I've had too many 'accidents' around white linens.) I first douche, well up into the Sigmoid to minimize the amount of waste in the lower colon and rectum. I hang the bag well above my head and use a clear hose and a flow indicator to note when the bag is empty. The common Green nozzle with generous holes speeds the flow, yet doesn't overly excite the rectum into urgent contractions.
I consume the entire 24 oz, usually within 2 minutes, insert a retention plug, get horizontal, relax, start timing and meditate for the holding period. Although my Hydrotherapist claims that the important chemicals for liver detox are absorbed within 12 minutes I hold the first enema for 15 minutes then expel, refill the bag and start the second enema, straight away. Time permitting I hold the second enema for 30 minutes under the assumption that the second enema is somewhat diluted by the expended residual from the first enema. That way I leave, satisfied that "I got it all.
Ben Greenfield Fitness and Dr. Larry Wilson recommend a holding period of 30 minutes while Dr. Balch says 15 minutes and flowingfree.org says that 12-15 minutes is sufficient.