I generally know a fair amount of things about Pokémon, but there are a few very basic things I don’t know, including when mega evolution activates, and how long it lasts.
Mega Evolution lasts until your Mega faints, unless the battle is over before it does, and in that case it with turn back to its original state when the battle ends.
Source: Experience
Hope this helps! :)
A Mega-Evolved Pokémon will only revert to its original state if it faints or the battle ends; if it is switched out, it will remain Mega Evolved.
Hope this helped!
Source: Experience and this
