For all intents and purposes, migrating from Magento 1 to 2 is a rebuild from the code aspect. There are some official (and unofficial) tools out there that will help migrate the core database tables and help to automatically convert magento 1 into magento 2 code but they're not comprehensive and there's still a lot of development effort that needs to go into it.
For that reasons, it's often been found that starting a fresh implementation (and migrating data) is the best way forward. That does mean that it will take about as long as a regular new site. So it's likely to be comparable to how long it took to do your Magento 1 build.
Obviously, the actual range of time will vary vastly depending on your scale, the partner you choose to work with and the complexity of build and how many off the shelf solutions are used compared to custom development.
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