What arrives in the first sync?

Every order, refund, product, variant and payment transaction your store has, back to its first sale. There is no window and nothing is cut off. On a long history the backfill runs in chunks and you can keep using the app while it finishes.

Lines 1 to 16 of your P&L, from GMV down to Net AOV, are correct as soon as that finishes. The margins below them fill in as you add costs.