There are many factors at play concerning retention enemas: fluid volume, fluid chemistry, body mass, blood chemistry, age, general health, etc. I have read in various medical articles the volume for retention enemas range in the 250 to 1000 ml amounts (1/4 to 1 qt approx); typically administered for constipation or stool softening. These enemas are usually isotonic solutions to closely match body pH thereby reducing the effect of electrolyte imbalance. It is further indicated these amounts can be safely held from 1 to 6 hours (again depends on all factors above).
Additionally, a single retention enema has a much less imbalancing effect than holding one after irrigating the colon with an enema series. This is because fecal matter acts as a buffering agent.
One study also suggested that homes equipped with a water softener pose a risk for increased sodium and chloride levels in the blood from large enemas with this type of treated water.
As with anything in life, moderation is the best course. Then again moderation can be boring...so if you are going to push limits, take it slow over time, pay attention, have a partner who can help, know your limits, do your homework, etc...