From not singing at all? My voice is terrible, I intend to start singing along to songs, doing breathing exercises and singing the do re me scale back and forth, how long before I can see a noticeable difference?
if u see you voice your smoking WAY too much pot lol
"Spin the middle side topwise. Topwise!"

"And there's Jimmy Page, the biggest thief of American Blues music"
I would see if there are any singing instructors around your area. They'll help you figure out any problems and improve. If there's not any, or their all too expensive (seriously, I find them the most expensive teachers) look for online tutorials and just practice. Sing at home, in the car, in the bathtub. PRACTICE.
Our first gig was somewhere in february, the 14th or something and our last one was 9 october. EVERYONE told me our singer haf improved a lot. We practice every friday, so we notice it only per gig.
But if you train and stuff, in a quarter of a year you could improve a lot...
It depends on what you consider noticeable improvement. I used to have pitch problems, so once I fixed that (probably took a couple months,) that was the first difference in my mind. Then there was getting higher notes. Then there was good tone. Then there was avoiding vocal fatigue. I'd say all in all, from the point where I sucked supreme ass, to the point where I can legitimately say I think I'm a good singer, was about two years. That's only my experience, though.
Everyone is different. How quickly you improve will depend on a lot of individual factors, from natural ability to how much you are restricting your throat and how quickly you're able to get that opened up.

For me, I have NO natural ability. As a result, I hardly sang a note from the ages of 12-22. Ten years. Even then, it took me about five years before I had the courage to pursue lessons. From there, after ten years of lessons, I finally consider myself a decent singer.

So... a few basic technique improvements that will lead to a noticeable improvement, to a total package improvement for chaingarden at two years, to a total package improvement for me at 10 years..... somewhere in there is your answer, I guess.

Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.
Record yourself. And I mean EVERY exercise you do. Depending on your age you'll see results within a month up to a year. I had singing lessons when I was 17 for a year. The first month I could alreayd hear improveements. Subtle ones, but they're there.

If you want results you need to look to the details.
Find a good vocal coach. Some of them record lessons and give them to you. If not, ask them if you can record the lessons. This way, if you can only afford to take 3 or 4 lessons you can have them on tape to keep practicing to.

When I started taking vocals lessons I noticed and immediate improvement. People who came to my shows were like you were good before but now you're really good. That was after taking 3 months of lessons.

It really does help. I've been taking lessons for about 2 years now and it still seem improvements from quarter to quarter. The more you practice the better it gets.

But definitely get some instruction on what things you are doing wrong. Take a few lessons, work on it. A few months later take a few more lessons to keep improving.
A guy I know, Joe O'neill, has the odd acoustic gig around Dublin (don't think he's been very active recently though), used to not be able to sing, only sorta "shout" sorta stuff (heavy blues style shout, not metal shout).
He took 6 months out to improve his voice as well as guitar playing, afterwards he has the best voice for his style of acoustic playing.

But the voice is an instrument, you'll see improvements straight away (like you will have with your guitar playing), hell I haven't learnt any voice excersises I only sing while I practice to learn co-ordination (gonna work on my voice ASAP though), and from just that I've had an improvement since I started.
My voice has been improving much faster than my guitar playing. Before I started trying to sing, I could sing along to a song in tune, but only after some practice. After about six months I can hear a song for the first time and be able to sing with it before it's end, and I can sing a melody without it hearing it. So I would conclude that considerable vocal improvements can be achieved much faster than similar improvement on guitar.