Count arrival and departure days separately. An overnight flight followed by a transfer rarely provides the same energy or flexibility as a full day already settled in the city.
Rome · YOUR QUESTION, ANSWERED
How many days should a Catholic spend in Rome?
Four or five full days is the practical minimum for a first pilgrimage to Rome. That allows the four major basilicas, which are not within one walking circuit (St. Paul Outside the Walls is about 3 km south of the center), a Vatican Museums visit of 2–3 hours, a Wednesday General Audience at about 9:00–9:30 a.m. if the Pope is in Rome, and a catacomb or Scavi visit, without putting everything into two exhausting days. Add time for slower walking or particular interests.
Sources & further readingVatican: the papal basilicasVatican Museums: visitor information
Official sources cover institutional information. Suggested pacing and itinerary choices are planning guidance. Check current arrangements for your travel dates.