You can collect a Free Chest every 4 hours and you can have up to two free chests ready to collect at any one time. Once you have collected your free chest, the 4-hour timer will restart.
Chests are awarded every 20 (silver chest) or 50 spins (gold chest), but only 1 chest can unlock at a time.
You can also buy chests in the shop. Purchased chests can be opened immediately.