Niantic has announced some adjustments to Mega Evolution in Pokémon GO including the duration changing from 4 hours to 8 hours and the cap for Mega Energy has increased from 999 to 2,000.

Updates to Mega Evolution!

Trainers, Two exciting updates have been made to Mega Evolutions, just in time for October Community Day!

• The duration that Pokémon stay in their Mega-Evolved form has doubled from four hours to eight hours.
• The cap for Mega Energy has increased from 999 to 2,000. This limit applies to each individual type of Mega Energy, not the total amount of Mega Energy you have across Pokémon species.

Stay tuned for additional updates in the coming weeks. Let’s GO!

Source: https://pokemongolive.com/en/post/megaevolutions-updates/