When the volume reaches a certain point (and that could be 2.5L or it could be 3L) I start to grunt/groan besides trying to pant/regulate my breathing.
As more solution flows in to me (and at this point it's usually "open clamp; count to 5 slowly; close clamp") I really start to vocalize. Not loudly though.
Once I determine that "if one more drop goes in I will explode" I try to hold onto that amount of solution for 5 minutes - groaning the entire time.
I have, on very rare occasions lately, been able to accept more solution once I thought I was at that explosion point, but I think that's because my body had gotten accustomed to the volume already inside and wasn't cramping as much trying to expel the invading solution!
The other day I was doing a coffee enema, and I had taken almost 4L (not something that regularly happens for me) and I rested like I noted above.
About 5 minutes passed, I and I found I wanted more. So I rose to the kneeling position, and added an additional 500mL of very warm water to the bag.
I was then able to accept the entire amount that was now available. My estimate of the total was 4.25L
I wasn't able to hold that amount much past a minute or two - perhaps if I had someone "distracting" me, it would have been possible to hold it a little longer.