My Mega-Evolved Pokémon returned to its regular form before the timer ran out. – Pokémon GO Guide

Trainers can only have one Mega-Evolved Pokémon active at a time. If you Mega Evolve a Pokémon while another Pokémon is Mega Evolved, the first Pokémon you Mega Evolved will turn back into its regular form.Mega-Evolved Pokémon will stay in their Mega-Evolved form for 4 hours, after which they will return to their regular form. You can see the remaining time on your Mega-Evolved Pokémon’s summary page.


Pokémon GO Game GUIDE