Well, I know what I've read (e.g., the whole length of colon is about 6 feet if stretched out). But in the body I believe it is somewhat shorter, like an accordion that is compressed. On my larger diameter tubes, I'm pretty sure of the tip location AND I've marked the tube with a marker and filmed it to see how far in the tube eventually goes in before I feel I have to stop.
I've read that sigmoids tend to be either "long" or "short." I have a "long sigmoid" colon. That is, the tube goes a long way up my centerline before it turns "down" and to the left to enter the descending colon. Once that turn is completed, I can feel the tube moving up my left side to the splenic flexure. For me, the entry into the splenic flexure occurs at an insertion depth of approximately 28 inches. It takes about another 4 inches to complete the turn and to gain entry to the transverse colon. My transverse colon is a long, gradual "U-shape." It is fascinating to be laying on my back and watch the tip slide below the skin from left to right in the transverse colon. It "disappears" on the right side and I assume that is as the colon tucks behind the liver.
At around 42 inches inserted, the resistance increases and I can feel the tip deep inside on the right side. I can twist the tube and keep a certain amount of pressure on and feel the muscles resisting insertion. But I can also feel the muscles suddenly release and the tube will suddenly slide in another 4-6 inches. Sometimes it helps to add a little enema solution in order to make that turn, but once that turn is made it is pretty easy to slide the tube to the next resistance point (which I assumed is the area that represents the cecum).
My thinnest tube is the one most easily "kinked" and it has taken experience to know when or if it folds over onto itself. When it is threading through with little resistance, it is really a neat sensation. And it may not be such a "straight line" through the colon as it "rattles around" between the walls. It may take me anywhere from 10 minutes to 30 minutes to thread that one through. But the sensation of pulling the tube out that entire length is pretty incredible (note: it is usually easier to thread the tube back through when I've done the first enema fill and clean out and approximately half of the second bag volume has drained into me).
The one-inch tubes are probably the easiest ones to thread through. They rarely fold over. They thread through the splenic flexure with relative ease and both "flow" into the transverse colon. However, the longer one (54 inches) is more of a challenge to make the turn at hepatic flexure. I think the tube has to make a turn of somewhere between 120 and 150 degrees to enter the ascending colon. If and when it makes that turn, I can slide the tube in and out by about 5-8 inch strokes. With lots of lube to ease the tube through and sliding it in and out that way, I'm not being ass-fucked by the tube, I'm being colon-fucked. I can't really describe the sensation of that tube sliding in and out throughout the length of the colon. If I can't make the turn at the hepatic flexure, even with the addition of the enema solution to help ease the resistance, 42-44 inches in is plenty far enough and injects the enema really deep into the colon.
Once into the ascending colon, there is much less of a sensation of the tube sliding in the ascending colon. If I can get the tube in that far and make the turn, I can usually insert the full length to the retention ball.
My current largest diameter tubes are 1.66 inches and they insert relatively easily to the splenic flexure. The sensations of these tubes opening, sliding past, and through the sphincter muscles at both ends of the rectum are really stimulating. However, a tube this large can be a challenge to get to turn because it is a balance between enough softness to bend and turn, but enough stiffness to be able to be pushed through the turns. It basically "seals" the lower colon section below the splenic flexure as it is inserted. I don't really have to guess where the tube is. And if the tube makes the turn at the splenic flexure, the flexure/tube combination is a pretty effective seal. Filling takes place in the transverse colon first with flow into the ascending colon and cecum. I can fairly quickly fill the upper colon with around 2 liters of solution before I get a full sensation. After that I usually dial back the flow rate. The colon will expand to allow the enema solution to flow back towards the rectum. I am mindful that the tube is so large that I cannot generally take the full 4 liters even with a large belly distention. Fully inserted, these tubes occupy more than a liter of volume.
Since those large tubes have a double retention ball design, not only is the tube a challenge, so is taking in the retention balls. They are large oval (egg-shaped) retention balls. The first retention ball begins roughly 8 inches from full insertion and each is approximately 4 inches long from beginning to end. So, on the shorter tube that is about 24 inches inserted and on the longer tube that is about 40 inches inserted before I reach the first retention ball. I cannot tell if the tubes are compressed very much (but even if they are, they will expand to their natural length).
On the shorter tube without the retention ball inserted, that places the tip just below splenic flexure for me. The longer tube (48 inches) puts the tip approaching the the hepatic flexure. That longer one can get around the splenic flexure but it is a really full feeling tube just by itself and very heavy. I have relatively little difficulty inserting that first retention ball because the diameter is a manageable 2.25 inches. On the short tube, that puts the total insertion length at 28 inches and just at the turn of the splenic flexure. On the longer tube, it's 44 inches and (I suspect) where the hepatic flexure turn can be a challenge.
Inserting the second retention ball is usually the real challenge for me, not because of where the tip of the tube is located but because inserting the second retention ball means the first ball has to be pushed up into the upper end of the rectum and into entering the sigmoid colon (that has to be expanded). That second ball doesn't really "pop in." However, if it slides in and is retained (a keeper ring really helps) it is an incredible sensation of fullness in my rectum. It takes a lot to relax and allow the retention balls to be held. The only thing comparable that I have are my largest glass butt plugs or an inflatable silicone plug that puts as much pressure when it is fully inflated.
Everyone is different and YMMV.