To earn PokéCoins without paying for real money, you need to interact with gyms, which you unlock when you reach level 5.

Gyms are rotating tower-shaped buildings (don't confuse them with PokéStops) that can be defeated by one of three teams. After defeating them, you must place a Pokémon in your inventory to defend this Gym from enemy players.

This is how PokéCoins work after the Gyms update in 2017:

• You earn 1 Pokécoin for every 10 minutes you have a Pokémon in a Gym;

• Coins will be automatically delivered when your Pokémon is removed from the Gym;

• There is a limit of 50 coins per day, which you can earn through all your Pokémon.

• This limit was initially mentioned on the official website, but this information was later removed.

• However, according to Silph Road, this limit seems to continue to be present in the game.

The game now works differently, because at first when you put a Pokémon in a gym you earned 10 coins per creature (with a cooldown of 21 hours each time you collected the coins). Now, in theory, you can easily earn 50 coins in the same time period.

However, if your Pokémon is in a gym for several days (earning more than 50 coins), it will only give you 50 coins in total when you are removed from the gym. This also means that if other Pokémon come back to you on the same day, you won't receive any extra coins, as you'll already have reached the coin limit for that day.

So it's a good idea for your Pokémon to come back from gyms often, rather than spending long periods of time in those places, so you'll maximize your profit.

It's also important to note that you can only have one Pokémon per gym, but you can put more Pokémon in other gyms. That is, you can place one creature per gym, having as a limit 20 gyms.