From level 5 and up, free coins are added to your Piggy Bank after every game.
The more you play, the more coins are added to your Piggy. However, if your piggy bank can no longer accept new coins, it means it is time to break it :)
Just make sure to break your Piggy before it expires, so that the coins don’t go to waste.
If you don’t see a Piggy Bank icon, it means that it has either expired or that you already broke it. Not to worry, a new Piggy Bank will appear in a few days.