A basic view of the "standard" layout of most anyone's innards.

The blue tinted area is the rectum and what would be filled and flushed in a "simple" cleaning out. This would need about a litre or less of water. Most anal sex and wrist-deep fisting would only stay in this area. Using more water than this gets into the sigmoid colon and now you're into a deep cleaning and looking at a lot longer of a process.

It's important to remember that this is NOT accurate to everyone; Some people's colons may not follow the placement or bends shown in typical medical illustrations like this. Some, for example, may turn in the opposite directions, may not bend down, etc.. This is an "idealized" layout.

Don't be surprised , as your hand is going far up your bottom's butt, if his plumbing is arranged slightly differently.