While I don't have a 50 year history of colon tube use like twobagger, I have used them for 20 years and have inserted then into a lot of people, both men and women I have played with in BDSM and medfet scenarios.
I happen to like stiff, thick colon tubes because you can easily feel where they are inside the colon which allows you to massage it around the curves. On insertion, you will probably feel areas where the tubes seems to get stuck a little because the tube is against an area that is curving. Follow my technique, and you should have a successful deep insertion.
Depending on the characteristics of the tube (diameter, stiffness) and your ability to know anatomically where it is in your colon and your skill in manipulating it, you could get more than 30 inches, but probably far less than five feet in unless the tube in kinked or folded--and most people don't realize when their tube is kinked or folded inside themselves. Be careful. Read posts on colon tube techniques. Here's what I do:
The method I use to insert is to be at least partially cleaned out first and be laying on your back--that is the position colonoscopy is performed in once you get to the transverse colon going on to the ascending colon and cecum. The reason to be on your back, and take a lesson from gastroenterologists here, is because on your back, your colon is on top of all your other abdominal organs. In the knee chest position, all your abdominal organs "fall" onto the colon inside yourself, and the weight of those organs can prevent easy passage of the tube deeper in the colon. Then when inserting the tube, use lots of lubricant so the anus doesn't get sore. Then allow some water to flow in and close the clamp. Now to insert the tube, I like to use the "jerking off my husband" stroking method as the tube is gradually advanced. What I mean is keep the tube moving in and out in two to four (or whatever is best for you) inch strokes and keep advancing the tube. I never found just twisting the tube to be as effective as the "jerking off" method, which is actually more or less what gastroenterologists do when they insert a colonoscope. They inflate the colon with air, we will use the enema water. Periodically, depending on how the tube feels inside you, add more water and close the clamp again and advance the tube more. Keep doing this until all the way in. WARNING: the tube will not go in as far as you think. Unless you have an abnormally tortuous colon or the tube folds over on itself inside you, you won't get in five or six feet of tubing. When the colon is described as 5 to 6 feet long, that is how long it is stretched out on a table, not sitting inside your abdomen. Consider the colon to be like the slinky toy. You can stretch it out far, but when it is at rest, it isn't that long. To demonstrate my point, look at a chart showing how the colon lies inside the abdomen. Then, take a rubber tube and trace that out on your belly. You'll see there is no way you are getting five feet of tube inside you without its kinking and folding. Three feet is about the maximum you will get. DO NOT force it, especially as you get the end of the tube in to the lower right quadrant where the cecum is. If you have a heavy, stiff tube (which is really what I like because it is easier to feel inside you and "massage" to where you want it to go) you can puncture your colon, which will cause a life threatening infection or you can damage the ileocecal valve if you try to force it into the ileum.