Oh, so they scheduled an appointment but you're not sure its been long enough for another culture?
I am not a vet, personally I would have them do the culture at the appointment. Worst case scenario they give you more meds. If it's an exotics vet and they deal with reptiles frequently I would definitely trust them.

If you were giving meds say every 72 hours or something then by the time the appointment comes around theoretically they should be "out of his system" by the appointment. Again, not a vet but that makes sense to me why you give doses every so often.