[RPG] If an elf is stable with 0 HP, do they regain all hit points after a long rest of 8 hours or 4

An elf receives the benefits of a long rest in 4 hours while using the "Trance" trait.

According to the 2017 update to the Sage Advice Compendium:

Q: Does the Trance trait allow an elf to finish a long rest in 4 hours?

A: If an elf meditates during a long rest (as described in the Trance trait), the elf finishes the rest after only 4 hours. A meditating elf otherwise follows all the rules for a long rest; only the duration is changed. This answer has been altered as a result of a tweak to the rules for a long rest, which appears in newer printings of the Player’s Handbook.

This ruling reverses guidance in the earlier version of the SAC, due to errata changing the rules for long rests.

Interactions between the "Trance" trait and long rests

A long rest is defined as:

... a period of extended downtime, at least 8 hours long, during which a character sleeps for at least 6 hours and performs no more than 2 hours of light activity, such as reading, talking, eating, or standing watch.

The elf's Trance trait is defined as:

Elves don’t need to sleep. Instead, they meditate deeply, remaining semiconscious, for 4 hours a day. (The Common word for such meditation is “trance.”) While meditating, you can dream after a fashion; such dreams are actually mental exercises that have become reflexive through years of practice. After resting in this way, you gain the same benefit that a human does from 8 hours of sleep.

Since the "Trance" trait replaces the need for sleep (which most races need in order to complete a long rest), the elf is able to satisfy the requirements of the long rest while in a semiconscious trance for four hours.

Sleeping is treated as being unconscious. Thus if an elf maintains consciousness throughout their trance, they are fully aware.

The big difference here is that the elf maintains consciousness. We do not at this time, have rules for what perception is like when a PC is sleeping (considering the use of unconscious, I'd guess it's automatic failure), so we can't necessarily determine what effect the semi-conscious meditation has on perception checks.

I'd say it's safe to assume that an elf can make perception checks (though generally for this kind of thing, passive is used), as normal with no penalty while meditating.