Is It Required To Take Cialis 5mg Everyday For The Desired Result?

My doctor gave me a 30 day sample dose of Cialis (5mg). He said I can use it only when needed, although the package says for daily use. He said it can last up to 72 hrs. I started it a few days ago and it seems to be taking affect now. I ve taken it 3 days in a row. If I stop taking it every day, will it continue to work only those days I take it, or will I need to keep taking it every day (or for a few days before I need it)?