I would say it is based on the experience you have with enemas.
In my case I’m lucky as I have a friend administer them.
When she first started we began with a small amount via a bag/ tube/ douche nozzle set up. And she started me knee/chest for a more straight flow into me.
We slowly began increasing the volume still using the douche nozzle.
That soon ended as the volume increased, retention was not effective. Retention was on my right side.,
As it turned out my friend brought a new inflatable nozzle which permitted me to take and hold more than previously. There was a true feeling of pressure after the coffee flowed through the inserted and inflated nozzle.
If I thought I couldn’t take more, I either expelled it or waited until the pressure that had built up started to ebb.
Currently I am given 2 enemas per session, each one 3.5 cups of warm coffee and retain for 10 to a maximum
of 20 minutes though most of the time it’s 10-12 minutes for each 3.5 cups.
We communicate so that if I feel anything I think might be undafe during the procedure she will end it.
Right now 3.5 cups per bag is my limit.