Immediate dentures are made prior to teeth being extracted. Chris takes some impressions and uses this information to construct a denture to replace extracted teeth. A dentist will extract the teeth and immediately place the pre-constructed denture.
Due to the healing of the bone structure and surrounding tissues the immediate denture will become ill fitting and loose. A denture reline will be required around 4-6 months post dental extractions.
It is also possible that even after the reline is done there will still be bone and tissue changes thus requiring further relines it really is dependent on the individual person.