A kitchen remodel is disruptive. A second-story addition, especially if you plan to "live in," is a full-blown military occupation of your home. The logistics, planning, and site protection are 10 times more complex than the actual building.
We had a client who was adamant: "We have to live here during the addition." I was... hesitant. We were adding a new wing over their main living room and kitchen. This meant that, for a period of about a week, their home would have no roof.
This isn't just a "tarp" job. We had to build a temporary, "weather-in" containment system. This involved erecting massive scaffolding and a temporary, load-bearing "floor" above their ceiling, and then a temporary "roof" over the entire work zone before we even removed the first shingle. We had to create a dedicated "construction only" access point from the outside, and the entire HVAC system had to be sealed off to prevent 100% of the dust from "invading" their living space.
It was an engineering feat in itself. The client lived, cooked, and slept just 10 feet below a massive construction site, and they never saw a single drop of rain or speck of dust.
My professional recommendation: If you're planning to live in your home during a second-story addition, your first question to the contractor isn't "How long?" It's "What is your site protection and logistics plan?" How will you manage dust? How will you protect us from the elements when the roof is open? How will your crew get in and out? The answer to that question will tell you if they are a true professional.