I'm afraid Celestia or Luna aren't eternal at all! Their lives as Alicorns began some time in the past when they died and were reborn.
Neither are they immortal. They live for very long but finite periods of time, ranging from Several thousands of years (Millions of human years) but When they pass away, they are reborn as some other sort of being, perhaps a different type of Pony,a Deva of a heavenly planet, perhaps a human or something beyond comprehension. (If lucky, they might be reborn again in a couple of eons since Flurry heart will bring forth a new age of alicorns.)
They're are not morally perfect ((no living being is)) Equestira may be a beautiful and happy place that lacks human passions and desires,but some of them are capable of ignorance and suffering.
The actual lifespan of an alicorn or a pony is unknown at the moment, But like humans and animals, they can also Die by Natural causes.
Equestria has a different time period. then other planes of existence.
for One Day in Equestria is one human Year in the human realm.
So a whole pony year in Equestria is 365 human years in the human realm.
((There you go.))
For each Kalpa, a Celestia and a luna is born till the next kalpa where they will die and be reborn again.