I usually find 2-4 days enough at any given full-service luxury property unless I am out sightseeing every day, all day. If on a beach vacation, 3 nights in a resort followed by about 10 nights in a luxury villa with direct beach access and a gourmet kitchen so I can putter and cook is perfect. I get really sick and tired of eating meals in restaurants all the time and not having a ton of space for the family to spread out. I'm never sick of vacation (when not actively "traveling") though so the balance of hotel and high-end rental is perfect.