This is so hard to determine because "time" in this case is subjective. Six months doing nothing but working on the novel? A year while simultaneously working and going to school? I've never devoted myself entirely to writing before these past three weeks, so I don't feel like it's fair to say that it took me years to write such and such a novel-- because if I hadn't been doing all those other things, it wouldn't have! (I'm sure everyone else is also dealing with real life!)
In any case I wrote my first novel when I was sixteen and it probably took me about a year, possibly a little less. Second novel was much quicker, likely around 5 months because it was very short. And not very good.

Third was a year and a half, fourth about a year. There were large breaks of time between each, in which I wrote short stories. And then I went to China for a year and then I spent a year getting married and settling down, during which time no significant fictional writing was accomplished. And I think that brings me up to now, number five, which I hope to have completed in about 4 months because my wonderful husband has agreed to let me focus on this full time. (Pressure's on!)