I don’t believe that there is a typical length for the insertion of a colon tube. Great care must be used when inserting a colon tube. You don’t want to pierce the wall of the intestine.
I have inserted 6" up to a 20" colon tube. One important point I feel I need to mention is that one should not try to insert the tube without first emptying the lower colon. What I do is to clean out my rectum, sigmoid section, descending column and part of the transverse column with several enemas and then use a long colon tube.
Valerie has answered the second part of your post very clearly.