The Legendary Bonus lasts for 1 hour when notified. When someone receives a Legendary Sidekick, the game shares a bonus to the chat group. Tap on the message to bring you into the game to increase your bonus. The Legendary Bonus increases your chances of getting a Legendary Sidekick from an egg. If you normally have a 12% chance, a 50% Legend Bonus would put you at a 18% total chance for a Legendary Sidekick.