Sorry, I tried to answer the question but I can be more clear:
It isn't just how long you've been on estrogen although that is a factor.
Your brain has to learn to associate some sensations with sexual pleasure. Some erogenous zones are inactive because of lack of use, particularly the nerves around the anus and around the prostate. Your brain has adapted to responding to penile stimulation because that's the input it's received, over and over.
If you stop focusing on your penis and start anally masturbating, ideally with some measures taken to put you in the right state of mind, you can gradually rewire your brain. Basically, you think sexy thoughts while touching an erogenous zone. Over time your sexuality itself can shift.
Everyone is different. Some people are born with a strong urge to shove things up their ass. For others, it might never appeal to them.