Before Celestia defeated her, that is. I feel like I read somewhere that ponies were afraid of going hungry because they couldn't grow food without the sun, and that's when Celestia reluctantly battled and banished Luna, but I can't remember where I saw that. Everything else I find makes it seem like it all happened on one day.

4473226
The flashback in season 4 showed that the first "eternal night" lasted all of five minutes.

The season 1 "eternal night" lasted only a few hours tops, the time it took to walk from Ponyville to the Castle of the Two Pony Sisters.

4473239 Cool. Thanks.

4473226
Pen Stroke made it an important plot point in Past Sins and Ms. Faust herself said that Nighttime Eternal would have doomed Equestria. A few comics touched on it as well.

Ti is right, canon wise the first "Eternal Night" didn't last all that long, while the second I think lasted no more than a few days. Headcanon wise its different.