How long does the first sync take?

It depends on how many orders your store has. The first sync brings back every order you ever took, not just recent ones. A store with a few thousand orders is usually done within minutes. A store with hundreds of thousands takes longer.

A long import runs in resumable chunks. Resumable means it can stop and carry on later without losing anything. If one step is cut short, the next one picks up where it stopped.

You do not have to watch a loading screen. You can use nouz while the import runs, and the progress stays in view. Each page tells you whether the data behind it is complete yet. Revenue lines appear first, because they need only Shopify. The margins below them fill in as you add your costs. You can also skip a setup step and come back to it later.