The Piggy Bank has a set cost and you'll be able to buy it whenever you wish to. However, because the prize will be getting bigger, the more crowns you obtain, the more convenient it will be to buy it.
To get the Piggy Bank's resources, first select the Piggy Bank's icon on the main screen of the game. Then, tap on 'Buy' , which will re-direct you to the App Store or Play Store where you'll be able to complete the purchase.