Based on the trailer, Legendary Raid Eggs will appear 48 hours prior to the start of the raid (in contrast to regular Raid Eggs which appear 2 hours in advance), but still only run for one hour.
If a player successfully defeats a Legendary Raid Boss, they will have the opportunity to catch a Legendary Pokémon. While Legendary Pokémon can be used to battle Raid Bosses and Gym Battles, they cannot be left to defend Gyms.
Pokémon GO Fest will be held on July 22, 2017 at Grant Park, Chicago. If players catch enough Pokémon during the Pokémon GO Fest Challenge Windows, the first Legendary Pokémon will be revealed in Grant Park; if the players in Grant Park defeat the Legendary Pokémon, that Pokémon will start appearing in Raid Battles around the world starting July 23, 2017.