toby,
The Malpensa Express takes ~52 minutes from Milano Centrale. I realize you've already booked a hotel, but if you want to reduce that time, you could consider staying near Milano Nord Cadorna station as the Malpensa Express also operates from there and the travel time is as short as ~29 minutes, depending on which train you choose. The area around Cadorna station also has lots of nice hotels, and it's also closer to the Duomo, Sforza Castle etc, and there are lots of restaurants along Via Dante (Dante is a wide pedestrian walkway, and a nice place to walk around in the evenings).
You can see all the solutions on the Trenitalia website. The trains with one change require a trip via Metro (or whatever) from Cadorna to Centrale, but there are some direct trains from Cadorna.
Be SURE to validate your ticket prior to boarding the Malpensa Express on the day of travel, or you'll risk hefty fines!