No one here is going to be able to tell you, there are too many factors that can change. Whom did you book through? What discount codes were applied? How long is your rental? Are you a Fastbreak member? What type of vehicle did you reserve? How early are you trying to pick it up? The answers to all of those questions factor into it. A half hour or an hour probably wouldn't be a problem, six hours and you're more than likely going to have to cancel and re-book.
Not to mention whether there’s a vehicle available, which may or may not be the case, given current supply levels (and, of course, the day you want the vehicle. January 21st may be a bit more likely than December 21st, in terms of availabiliy.