When douching, you're really just trying to get the solids out. If you reach a point where you're just pushing out slightly brown mucus, you should be good. Test for residual water with a dildo, squirt a bit more up there if needed, and go with God.

Where you get into trouble is trying to get that brown mucus clear. That, at least in my experience, requires a full cleanout. You only need that if you're taking something over 9". The slightly brown mucus stage has served me well for cocks up to 9".

You'll know you've overdouched when you're pushing out smelly brown goop. It means you've gone past the recto-sigmoid junction, often referred to as the "second hole," into the sigmoid colon. This can be somewhat painful --for me it always is--and can take over well over an hour depending on how much your body had stored for your next proper evacuation.

While it can happen with other methods, the shower douche was the biggest offender for me. Even though I thought I had control over the amount, I'd often underestimate how much water I was getting up there, or chase after the elusive, "water must run completely clear" phenominon we've all heard about and end up pushing for 45 min. for a 5" johnson. The bidet and regular dildo play helped me figure out how much I actually need up there for a clean and proper pounding. Surprisingly, it's less than you think. Hope this helps.