You're exactly right, jcw. Citrus notes are the lightest and the shortest lived and certain very heavy molecules, including spice and musk notes can last for days. Florals tend to be somewhere in the middle, although iris (orris root) is very long lived. And the jasmine in A La Nuit, for instance, lasts for days on fabric. You may know that perfumes are traditionally composed with top, heart and base notes, and if you look at the note pyramids listed for many scents in the Basenotes Directory, you'll get a rough idea of what chords will hang around for more than the first hour, as you say. Some members include attention to how long-lasting and coherent perfumes are as they dry down. Some perfumes have spectacular openings and boring drydowns; some perfumes open with strident topnotes and settle into something beautifully wearable. Heavy orientals like Serge Lutens' Muscs Koublai Khan and Cuir Mauresque last forever. Which according to some is a mixed blessing.