The minimum time between 2 procedures can be 2 to 3 months with regards to the healing of the donor area. But usually, people go for the second procedure after 6 to 12 months at least.
In the past, FUT and then FUE was suitable for patients with less donor. However, with the improvement in body hair extraction and beard extraction, even FUE can provide a good coverage. Also, if fine and sharp punches are used during the extraction then there would be a possibility of extracting a higher number of donor grafts from the index donor (all the while maintaining donor aethetics).