There is no legal or canonical answer. The Church simply requires us to attend Mass, not parts or percentages of Mass. If we are late for Mass, the question is, why are we late?
If we are late because of circumstances beyond our control or because of essential responsibilities, then we still fulfill our obligation. If, however, we are late because of a lack of caring about getting to Mass, then we certainly have not fulfilled our Mass obligation in any real spirit of the law.