Lots of variables in that question. It all depends on the gauge of the yarn, size of needles, and -- of course -- the size and complexity of the sweater. Add to that how much time you make available for knitting, and....well, you get the idea. Who knows?
My first sweater was a basic v-neck pullover out of pretty chunky yarn. If memory serves, it probably took me about a month of pretty part-time knitting to finish it up.
My second sweater, which is basically as basic, but made from a finer gauge yarn is taking me forever. I think I started it in September, and I'm finally working on the last sleeve. Though, because of the pattern on this one, sometimes I've felt like I've done more ripping than knitting.
My advice is relax and enjoy. You'll finish when you finish. And, it'll be perfect!