50 min with bus (bus 73 + metro or bus 94 depending on the day/time) or 30 min by taxi.
Direct train from the airport to Cadorna train station. It takes approx. 50 min.
Airport shuttle from Orio al Serio to Milano Central Station. It takes approx. 1 hour + 15 min with underground (Green Line – Get on at Caiazzo, get off at S. Ambrogio)