This will all depend on how many islands you plan to visit during your visit to the Azores. In general, I would say 3-4 days for Sao Miguel, and 2-3 days for most of the other islands will give you enough time to see the top highlights of the islands. Since you probably wont be going to all islands during a trip, I like to think that a 10 day trip is the perfect way to see Sao Miguel and a couple more islands in the Central (or Western) island groups.