Interesting results immediately, I thought it would take longer. This is what's happened so far.
My full voice range has shifted. It's shifted up slightly, by about a tone. I'm able to hit an A as my highest note (5th fret, high e-string). On the other hand the lowest I can hit with conviction is an Eb (half-step below the low E string). This is interesting, I knew that my voice could drop quite dramatically after a big night out, smoking in excess of a pack in one night, but didn't realise that there was a possibility that smoking was having a constant effect on my voice.
I can control my breathing a bit better, enabling me to sustain notes for longer and singing longer passages without breathing. It doesn't feel like I'm breathing deeper than last week, and I'm probably not - it's only a week. But I can let out air in more controlled bursts, allowing me to sing for longer. I've also gained more control over the higher range of my voice.
When I sing, the result is more consistent. Usually it would vary depending on how recently I'd had a cigarette. So say singing 4 mins after a cigarette feels different than singing 30 mins after a cigarette. I'm not saying that there is a change in quality between the two, but that the feeling of singing is more consistent. This makes it easier to practice because it's always the same environment for my voice.
My mouth is not as dry, so no coughing randomly, or weird tickling things (I guess cigarette gunk) in my throat when I sing above a certain note.