If a demon loses a part of their body it will continue to "live" for some time. However eventually the main part of the body will detect that it has been truly separated and commence using energy reserves to grow back what was lost.
There have however been documented cases where a limb was grafted onto another demon, in which case the main body is sent conflicting information about the lost limb. As long as the part of the body continues to be used then it will be nearly impossible to regrow it, no matter the distance.
This is considered EXTREMELY invasive and it is absolutely viable to use favors, friends, or even the support of a Master to retrieve the lost body part.
Additionally the grafted body part has been known to influence the demon that it is now a part of, so it is not advised to attempt this.
Records show that a Demon Master once grafted a hundred different demonic parts onto their body, and gradually went mad and had to be completely destroyed by the other Demon Masters. See Fig 34.
[Graphic illustration of a demon being ripped apart]
-Textbook on Advanced Demon Biology and Application, for use by 22nd year students