Once you have Mega-Evolved a Pokémon, you can evolve it without Mega Energy following a resting period that may last up to several days.

To Mega Evolve a previously Mega-Evolved Pokémon without using any Mega Energy, you will need to wait for the resting period to expire.

You can choose to Mega Evolve a Pokémon before the resting period ends, however, this will require Mega Energy. The less time remaining in the resting period, the less Mega Energy will be required to Mega Evolve that Pokémon.