3 full days for Lisbon, and a 4th if you want Sintra

Day 1: Alfama and the castle, past Portas do Sol and Santa Luzia

Day 2: Baixa, Chiado and Bairro Alto, funiculars for the climbs

Day 3: Belém in the morning, sunset at Senhora do Monte

Two days works only if you skip Sintra and halve Belém

Five days adds Cascais and one longer trip, such as Évora

In August or rainy October and November, add a day to the plan