Mega evolve once a week
Search for "mega1-" in Pokémon GO to see which of your Pokémon are already Mega Evolved.
- mega1 (Base): reached right after the first mega evolution. Normally costs 100-400 Mega Energy depending on type. Mewtwo costs 7,500.
- mega2 (High): reached after 7 total mega evolutions of that individual Pokémon.
- mega3 (Max): reached after 30 total mega evolutions of that individual Pokémon.
- mega4 (Super Max): costs 5,000 Mega Energy, and only applies to Pokémon that have a Super Mega form.
So you cannot reach Super Max just by mega evolving over and over, the most efficient way is doing many Super Mega raids to gather the Mega Energy. That hopefully gives 400 Mega Energy on average per raid. You can also collect Mega Energy by walking with a buddy of that type, which gives 5 Mega Energy per kilometer.
I usually mega evolve everything matching the search string mega1-2 once a week, without spending any Mega Energy. After 30 weeks they reach level 3.
You can mega evolve more often, or raise the mega level faster, by "paying" with Mega Energy, but I spend almost all my Mega Energy on the first mega evolution and on Super Mega, not in between.
There is also a "Successor" medal that requires 1,000 mega evolutions for platinum. If you do 20 mega evolutions a week, you get the medal in just under a year.