It depends on what time of day you're flying in and which passport you hold; and if you have checked baggage.
I recently flew in to MXP T1 from MAN on BE in the early afternoon. A flight had just arrived from Asia or the Middle East and there was a very long queue at immigration–which looked as if it could take the passengers at the back of the queue at least an hour to clear. The area was packed. (Having an EU passport–for the moment at least–allowed me to get through in a matter of minutes through the appropriate channel.)
In my experience MXP getting through MXP can be painfully slow at certain times of the day (such as the early afternoon). It always seems as if there are fewer immigration officers and baggage handlers at work during lunch and dinner times; but have no real evidence to back this claim.
Trains from MXP to Milano Centrale depart every 30 minutes during the day and will often take the best part of an hour to arrive.
From touchdown you could make Milano Centrale in less than two hours (if all the stars line up)–but it could take well over three (or more).
If it were me I'd calculate the time taken for the worst case scenario–then double it! If you get to Milano Centrale early you can always use the spare time wandering around this magnificent building.