Re-using without massive refurbish costs is their ultimate goal, hence the assumption. If things go according to their plan, they are going to have turn-around

Re-using without massive refurbish costs is their ultimate goal, hence the assumption. If things go according to their plan, they are going to have turn-around of single-digit hours. Avoiding massive refurb is one of the reasons why they are not keen on dropping their rockets into salt-water for several hours. The Shuttle SRBs were really only nominally reusable. SpaceX is going for actually reusable. A Falcon 9 lower stage also isn't a spaceplane covered in very expensive and fragile tiles, sitting next to a rocket. That helps.