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 8 hours, after which they will return to their regular form. You can see the remaining time on your Mega-Evolved Pokémon’s summary page.