To go from living, breathing necromancer to lich can take as much as a decade to complete. First, upon death, the necromancer's remains must be carefully mummified and preserved, and placed in a special sarcophagus while rites are carried out. The mumification process may take anywhere from 120 days to 3 years, depending on the importance of the individual and the level of detail involved.
The spells and enchantments that form the lich can be completed in thirty days, but require much of the temple's stored energy. Typically the process will be spaced over a number of years.
The completed lich is fully autonomous and no longer requires the temple or an otherwise enchanted area to survive. The soul, complete with the memories and abilities of the original necromancer, is fully installed in the lich.
They can live for centuries and often continue to carry out roles within the temple community. Liches are also typically more powerful than living Necromancers.
Typically only moderately high-ranking priests become liches. Because the process relies on other necromancers completing the rituals after the death of the individual wishing to become a lich, it is uncommon for any other than community leaders to be given those rites.