The honest answer
Sri Lanka looks compact on a map. Distances are not. A private chauffeur-guide makes the joins human, but you still cannot do Sigiriya, the train, Yala and Galle as four frantic trophies in five nights.
Seven days
Choose two regions, not four. Culture and coast, or hills and a park, are both workable. Trying to “do the island” in a week usually means hotel corridors and a tired last morning.
Ten days
This is the length we recommend for most first visits. Cultural Triangle, Kandy and tea country, a safari, then the south. See the 10-day itinerary outline and the public Signature journey for a day-by-day sketch you can still change.
Two weeks
Add a second park only if wildlife is the point. Otherwise spend the extra nights doing less: another evening in Ella, a third beach day near Mirissa, or a slower climb at Sigiriya.
If you already have dates, a tailor-made itinerary is more useful than a generic packing list. Families should also read travelling with children.