I'm going to be visiting Tuscany for 2 weeks in august. It was cheaper for me to fly into Milan then Rome even when I added in a full fare trenitalia ticket. I plan on purchasing in advance anyway to get a cheaper rate. I land in Milan at MXP at 8:50am. Based on my last flight into Milan I could be in the city center at Milano Centrale around 10:30-11 range. I've never done anything in Milan and want to spend some time in the city before boarding a train to Siena. How much time should I stay in Milan before boarding a train? I want to see the Duomo and climb it and maybe a couple other things. Is 4 hours enough and i could possibly take a 3:15 train out of Milan?
I think that's possible. Once you get to the center, Milan is not a difficult city.
If you have time, Castello Sforzesco is impressive - and nice grounds for a walk after a long flight. The Galleria is near the Duomo. If you are interested in seeing Leonardo's Last Supper, you need a reservation time in advance, and it may not fit your schedule.
I actually think Milan is worth more than four hours. I think it is often overlooked by many. But to specifically answer your question, you can do it in four hours. The Duomo plaza is two stops from the Centrale train station and there is luggage storage at the station.
The Duomo is one of the Subway stops in Milano and is directly connected to Centrale. Attributing a few hours to the Duomo and in the climb will probably suffice (depending on your interest in the subject matter). The Galleria is on the same square as the Duomo.
Are you headed directly to Rome from Milan, or somewhere else? I ask in case you are limited in your train options.
Unless you are taking only carry-on, I think 2 hours from MXP to Milano Centrale is the about as early as you're likely to arrive. If you have to wait for luggage, or if there's any flight delay, it would be later. You will need to allow for wait time for the train from MXP, and time to check your luggage at the Milan Central, then to pick it up before going to your next train. My experience with left luggage in Italy is that you need to allow some time - there's often only one staff person and there may be someone (or more) ahead of you, or he may be on a comfort break.