For this rocket, the process for figuring out if they can use it and how was very complex. This rocket was originally flown for the CRS-8 mission on april 8th, 2016. So it took them just shy of a year to ensure that this rocket could be reflown, and that they felt comfortable putting a customers payload on it. They also fired it on the ground many times, and I would imagine checked it for every imaginable component failure.

How ever, for the future, only time will tell how confident they will be able to be in the re-usability of their rockets. Most likely they will retire this rocket, as with the one that was first landed, and put it on display somewhere. After they research it, of course.