That depends on how many tasks you have in your Remember The Milk account! If you're using an iPhone and have a lot of tasks in Remember The Milk already, you might wish to set up the app using a Wi-Fi connection (rather than your phone's mobile connection) to speed things up.
The initial sync needs to transfer all your tasks and notes from Remember The Milk to your device. To make things a little faster, by default it will only transfer completed tasks going one month back. If you'd like to change this setting, once you're set up you can find this option on the Settings screen.
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