I was a classically-trained vocalist for 15 years before starting T.
I started T with 3-weekly IM injections at 1/2 dose for the first 3, then full dose thereafter. My voice drop occurred gradually, not dramatically - from about the 3 month mark. I noticed before that, that my singing voice started to struggle - at first with the higher notes, then the higher end of the mid-range notes that I used to hit effortlessly. By "struggle", I mean that I would open my mouth to sing the note, and nothing would come out at all - or, if it did, it was cracking and hoarse.
On the other hand, I began to easily hit notes in the lower baritone range nearly straight away - and after a year, I could sing in bass range. However, my entire upper range is gone - I struggle to sing high tenor notes. Granted, I haven't practiced or pushed myself as hard as some guys to do regain their abilities, but I have noticed that if I sing for a couple of hours - hitting the high notes becomes a little easier.