I agree everybody experiences changes differently. I have certainly seen someone who has been on t for several months but his voice still hasn't dropped significantly.
For me personally and from what I heard for other guys, voice change is one of the very first noticeable changes. But if you see your relatives quite often, the changes may be so gradual that they may not even notice. I have an uncle that I haven't talked to in months, the first time I spoke to him on the phone after I was on t, he asked about my voice and coincidently I was having a cold at that time, so I just told him I was sick. The second time I talked to him, he didn't even think twice and ask me about my voice anymore.