It's much more dependent on what the current pitch of your voice is. After trans guys go on T, their voices typically drop about an octave, over a period of months to years.
Usually the changes are pretty fast to begin with, and they're likely to be accompanied by one's voice "cracking" or "breaking" at times. Then the pitch seems to plateau, but many guys report continuing changes over at least a couple of years -- just much more gradual, so there's not so much drama, and the change is less noticeable.
Also, the "one-octave" estimate is relative to your natural pitch. There is, unfortunately, no reason to think that if you force your voice into a lower range, that this will carry over to how low your voice ends up -- and many of those voice-lowering exercises can damage your vocal cords.