How long must I have worked for the company before an ICT transfer to Sweden?

Migrationsverket requires that you have already been employed at the sending company outside the EU for at least three continuous months before the transfer begins. This shows the move is a genuine internal transfer rather than a new hire.

You also need to remain employed with your home-country employer for the duration, because the ICT permit is built around returning to your original job once the assignment in Sweden ends.