The rugged island of Sardinia deserves at least a week to do it justice, but you may want to spend a long weekend at one of its attractive little seaside resorts.
You need to spend a week in Sardinia when you visit. 
mnli hypothesis: You need to spend a week in Sardinia when you visit.  premise: The rugged island of Sardinia deserves at least a week to do it justice, but you may want to spend a long weekend at one of its attractive little seaside resorts.