Everyone's plumbing is different, so I don't think anyone will be able to quote you a definitive answer about how long it will take you.
Are you able to get thinner toys like a 1.75 in by 18 in double dong deeper than 12 inches?
Using a 12 and 18 in double dong at the same time was the way I was able to train to take larger diameter toys deeper.
There are several threads about depth in this section "Howto & FAQ". Have you read them?
http://www.fistforum.com/howto-faq/did-i-pass-the-sigmoid/http://www.fistforum.com/howto-faq/how-to-train-for-longer-length/http://www.fistforum.com/howto-faq/how-to-go-deeper-in-anal/more can be found with a Google search for
depth training site:fistforum.com
Also the following is often referred to
http://www.jlubejack.us/goingdeep.html