The rules for Pokémon go's mega evolutions have changed over time. There doesn't seem to be any one place in the publisher, Niantic, states all the rules, mechanics and benefits for mega evolutions.
The question is really: what is the complete set of rules that governs how mega evolutions work in Pokémon go?
This is partially repeat from This is the Dave's answer but with some extra detail, subheadings and updated for the late April 2022 Mega Evolution changes. And thankfully, more of this information is now stated in-game, particularly the Mega Evolution levels.
List of current Mega evolutions, their initial Mega evolution cost, and their types if they have changed from their base forms:
100:
200:
300
Mega evolution cost
The initial Mega evolution cost is either 100, 200 (most common) or 300 (Pokemon requiring 400 candy to evolve and legendaries). After a Pokemon has previously Mega Evolved, a Mega Evolution timer starts. At the beginning of this timer, the Mega Evolution cost is reduced by a factor of 5 for that Pokemon only. For example, a previously Mega Evolved Beedrill has a cost of 20, but all other Beedril still have a cost of 100 unless they have also Mega Evolved previously. This reduction remains even if the Pokemon is later traded. Over the timer, the cost gradually decreases over time till the timer finishes, at which the cost then becomes 0 (i.e. free Mega Evolution). The duration is 7 days at the base Mega Evolution level (more on this later).
Earning Mega energy
Mega energy can be collected for all Pokemon who can currently Mega evolve. The methods of collection are:
In-battle Mega evolution effects
All Mega Pokemon have altered Atk and Def which give overall boosted stats (currently all Pokemon have both Atk and Def stats improved, though when Mega Diancie gets released, it will likely lose Def). The Stamina stat will not change.
If a Mega evolved Pokemon is on the field in a raid or gym battle, all its and allied Pokemon's moves that share a type with the Mega evolved Pokemon will receive an attack boost until it leaves the field i.e. switches or faints.
Mega evolved Pokemon are useable in all modes except for (1) PvP battles against other humans and (2) being placed inside gyms. They can be used for training and against Team Rocket, and can be used to beat gyms and raids.
Outside-battle Mega evolution effects
Whilst you have an active Mega evolved Pokemon, all Pokemon caught of the same type as the Mega Pokémon reward +1 additional candy. +1 candy will also be rewarded when captuing a raid boss regardless of its type. A icon will appear on your profile on your friends' friend list, indicating that you have an active Mega evolution. If the Mega evolution Pokemon is your buddy, this Pokemon will be visible to other players in its Mega evolved form when they see your profile.
Mega evolution duration: 8 hours
Max Mega energy that can be stored per Pokemon: 2000
Mega Evolution levels
This is new as of late April 2022. If you Mega Evolve a Pokemon enough times, it moves up a level. There is a base level and two levels above that. Moving up a level can:
I'll have a go at answering some of this, I think this is incomplete however.
Initial mega energy costs for Pokémon are 200 or 100. Once that particular Pokémon has evolved, it is unlocked and the cost of mega evolving further times is reduced to 40 or 20. Each species has their own energy.
200 candy needed for first mega evolution, then 40; Abomasnow, Amphardos, Blastoise, Charizard, Gengar, Gyarados, Houndoom, Venusaur. Manetric added March 2021
100 candy needed, then 20 Beedrill, Pidgeot
Only the Pokémon you evolved has the mega energy cost reduction, not all Pokémon of that species.
Source: 3
Walking with your buddy earns 5 Mega candy every km, but ONLY if that Pokémon has Mega evolved before. The mega energy is given when the buddy pokemon gives candy, this depends on the Pokémon's candy walk distance (1,3,5,20 km). If the distance is 3km, it will earn 15 Mega Energy every 3km but not before 3km is walked. 6 This means that it will require 9km to evolve the first list and 4km for Pidgeot or Beedrill. 7
Your buddy doesn’t have to be the same Pokémon that Mega Evolved, any Pokémon in the evolution line will earn Mega candy after you have Mega evolved once.
Mega Pokémon are active for 8 hours.
Mega Pokémon give you +1 candy to all Pokémon caught of the same type during the time it’s active.
Mega Pokémon will give +1 candy on the catch of a raid boss, regardless of your mega type, during the time it's active.
When in a raid or gym battle, when a Mega evolved Pokemon is on the field, all allied Pokemon's moves that share a type with the Mega evolved Pokemon will receive an attack boost until it leaves the field i.e. switches or faints.
Mega energy is gained when battling Mega evolved Pokémon in Mega raids. The speed at which you defeat the Mega raid is linked to how much energy you receive. Mega energy is also gained in completing research tasks.
Mega evolution energy is capped at 999.
Additional energy gained from gyms in a set time: https://www.reddit.com/r/TheSilphRoad/comments/miv2i5/what_are_the_breakpoints_for_mega_candies_when/gt6w4f5
Time to beat the boss (Amount of energy) 0s-0:29 (90 energy) 0:30-1:14 (80 energy) 1:15s-2:29 (70 energy) 2:30-3:44 (60 energy) 3:45-4:59 (50 energy)
Some speculation about rules:
Beedril is gaining energy from spinning gyms 4. This is at a rate of 5 energy given on average every 8 spins 5.
