I ran 3 days about a month ago and got to #1 overall (in free) for "My Temporary Life". When it went paid, I hit #9 overall, so I was very happy.
The days I ran free were Fri, Sat, Sun. This time I am going to try Wed, Thurs, and then hit Fri, and the weekend. Fri is quite often a strong sales day and I think I can build up enough inertia to hit the ground running.
Thanks for asking the question, I hope you get more responses, to me it's one of the real puzzles with the program-which days and how long do you run?
Having had a few more months experimenting with promos I've come to the conclusion that two days is the optimal amount of time for a promo - enough time to get your book traction on the 'hot list' but a short enough amount of time to make sure your giveaways do not completely fall off a cliff.