I don't know about Delta, but American will put FF reservations on hold for a month at a time.
I had a dickens of a time getting 5 FF seats on the flights I wanted when I made reservations in January for travel at Thanksgiving! However - having been ticketed since Feb. - I recently changed my return flight to 2 days later without any problem or penalty.
I agree with Lewis about saving miles for the more expensive routings. In our case, my DH has lots of FF miles, and we're doing a stopover on the way to Orlando. The great thing about FF tickets is that you can fly open-jaws: into one airport and out of a different one, as well as the ability to make a stopover along the way.