I’ve found that oatmeal and honey goat milk soap is the most mild. I actually tried just a bit ago (see photo in my latest album) to make a very strong solution by swishing and dipping the bar of soap into the bag for a good few minutes. It was strong, but not unbearable. I got a cramp and a wave of peristalsis about 3 liters in, but I was able to relax and breathe through it to continue and finish the bag. I couldn’t retain it at all, though; once I heard the lovely sucking sound of a job well done, I had to expel immediately.
I’ve comfortably held milder solutions with this same goat milk soap for much longer, but I agree with @Annie2 that Bronner’s is quite strong and causes a very strong sensation of urgency. I’ve found that about 30ml max in a gallon is comfortable, less when using the ones with essential oils. The almond in particular is not fun, but eucalyptus and lavender have been fine for me. I keep a bottle of the unscented around just for enemas, and the peppermint is a special treat since it leaves you tingling, but again it’s strong. I’ve definitely used too much soap in the past and it left me dry heaving until I was fully rinsed out again, which can be a difficult situation to relax in, further compounding the bad effects of too much soap. That’s not personally super fun for me, so I avoid it.
With all that said, I’ve done 2-3L M&M enemas a dozen times, and those typically have fewer lingering effects once properly rinsed out. They definitely make me sweat and feel like I’m dying though, so I only feel like one in a very special occasion. I’ve found colonics to be much more enjoyable and easier on my body, so now that I have a few decent ways to take them, that’s been my go-to for days when I have more time to enjoy a longer session.
Lastly, make sure to stay hydrated! I’ve found that a stomach empty of food plus continuously sipping juice and water or a hydration sports mix with electrolytes also helps me feel great after a session instead of worn out.