I believe this has to do with your proximity to the dwarven civ of which you are a part. That is, if you snuggle up right next to some other fortress, you'll get migrants really early, while if you're sitting in the middle of the tundra at the other end of the world, migrants may not make it for more than a year. The fastest I've ever seen was in the first winter, while the slowest was almost two years (I abandoned second winter due to, um, BUILDING AN ENTIRE FORT WITH SEVEN FREAKING DWARVES).I haven't actually attempted to verify any of this; it's just my recollections over dozens of embarks.