How long should I plan for the time between landing in Milan and reaching the central train station. My flight arrives from Frankfurt via philadelphia at 10:35 and the train leaves at 1:55
It might be close, especially if your flight is late, but it's possible.
Are you landing at Malpensa? If so, take the bus shuttle (two choices, Malpensa Shuttle and Malpensa Shuttle Air Pullman) - both drop you off at the east side of Central station. 40-50 minutes depending on traffic, plus the wait for the next bus (there should be one about every 20 minutes). You can buy your train tickets at the kiosks on the first floor of the central station, or pick up pre-paid tickets at the same spot.
You will clear passport control in Frankfurt. I'm not sure about the wait for luggage in Milan (I only carry on) but I'm sure other posters will help out with this.