That's the question. You can say “prescribed amount,” or anything else. For me, it's when no more water will go in (bag or can at a reasonable level). If she has a lot of gas, it's sometimes not much to start. If it's her 3rd or 4th enema, 2+++ quarts. I have never had a receiver unable to take at least 2 quarts, if she's had a small enema at first to empty her rectum. It's easy to check.