Three days will give you a sense of the region, five days will allow you to include a visit to the shrine at Fátima and the Convent of Christ, and a full week will give you enough time to cover the major attractions as well as explore the countryside. With additional days you can easily extend your itinerary to include Évora and the Alentejo or head north to Coimbra and the Beiras. It's best to explore these regions by car, unless you have a great deal of time and patience: trains don't serve many of the most interesting towns, and bus travel is slow.
If You Have 3 Days
Start with a visit to the imposing monastery and palace at Mafra, then head for the coast, with a stop at the resort and fishing village of Ericeira. Continue north along the shore to Peniche, with its imposing fortress. Head inland to spend the night in the enchanting walled city of Óbidos. The next morning continue north, stopping at ceramics shops in Caldas da Rainha. En route to Nazaré, tour the church and cloister at Alcobaça. On your third day head inland to the soaring, multispired monastery church in Batalha. Return to Lisbon.
If You Have 5 Days
Follow the three-day itinerary to Batalha, and then continue north to Leiria and its hilltop castle. Take N113 east to the A1 and drive down to Fátima, one of Christendom's most renowned destinations. The next morning, continue east on N113 to Tomar, an attractive town dominated by its hilltop convent. Later in the day, follow N110 south and then take N358-2, a scenic road that follows the Rio Zêzere, to its union with the Rio Tejo. Just west of their confluence, on an island in the Tejo, is the Castelo de Almourol, one of Portugal's finest castles. From here, follow N118 southwest along the Tejo, stopping in Alpiarça to visit the Casa dos Patudos, a large country house containing the art collection of its former owner, and in Almeirim to see the winery at the Quinta da Alorna. Spend the night in Santarém, an important farming and livestock center with many fine sights. The next day return to Lisbon with a drive along the Tejo on N118 through the region of marshy plains known as the Lezíria.