In the past, the free dining packages have had a minimum number of nights and ticket days required. I think last year free dining required a minimum three night/two-day ticket purchase to qualify for FD. The requirements could change for this year and I don't think they've announced any free dining offers at this point.
Almost every FD offer I've seen has had a minimum of 3 nights. Many times when Disney announces a FD offer it will include an example of "book a 4 night/5 day stay at xyz resort..." which makes it sound like that is the minimum required, but somewhere in the offer the actual minimum will be mentioned.