Technically speaking, your large intestines are about 5 feet long, so that's your maximum depth.
But I think what you're talking about and experiencing is that the average rectum is only about 6-8 inches in depth before it makes a turn and becomes more properly described as part of your large intestines.
Some people who are into fisting have worked to straighten out their sigmoid colon and shifted their descending colon more towards the middle of their gut, allowing someone to go in about 12-15 inches (the length of a forearm).
Unless you're really trying to get into the realm of fisting, if you feel like your dildo is "bottom out" (hitting the end of the rectum) your best bet is to maybe wiggle it towards your left side and maybe you can start to work it into the part that becomes the sigmoid colon. Eventually you may rearrange things and let you go much deeper. But the down side is that you could also rupture your colon and then you're looking at possible septic shock and death, surgery, maybe a colostomy bag, and a very likely end to your anal adventures.