Minimum of the Sunday Obligation
What are the latest arrival and earliest departure requirements in order to fulfill your Sunday Mass obligation?
The traditional manuals of moral theology prescribed, as a minimum of attendance in order to fulfill the Sunday Mass obligation, the time from the offering of the bread through the priest's reception of the Precious Blood in his communion. This was because those limits represent closely within the liturgy the theological essence of the Mass: the offering of the matter, its consecration, and its reception by the sacrificing priest.
Even though this minimum would fulfill the obligation strictly, it is not at all what the Church intends for its members. We should attend the "whole" Mass and come late or leave early only when there is some urgent necessity to do so.
Nowadays one hears sometimes that the minimum would include the reading of the Gospel. There is a good reason for this liturgically, since the Gospel is the most important element of the Mass for preparing the faithful to celebrating the sacrifice, but it would not be reasonable to say that someone who attended at least from the offertory to the priest's communion did not fulfill the obligation. But I reiterate: normally the obligation includes the whole Mass, not just its most essential parts.