You can run Legacy Dungeons in World of Warcraft as follows:
Normal Difficulty: Multiple times per day, no lockout.
Heroic Difficulty: Once per day, resets daily.
Mythic Difficulty: Once per week, resets weekly.
In World of Warcraft, Legacy Dungeons drop loot at a level appropriate for the dungeon when it was current content. Additionally, certain rare items like mounts, pets, or transmog pieces may also drop, which are often the main attraction for higher-level players running these dungeons.
All items on the loot table are eligible to drop, not just the ones specific to your class. This allows for a greater opportunity to collect transmogrification appearances from older content.
Legacy Raids, which are from previous expansions, can often be completed alone due to increased level caps. However, some raids from the most recent expansions are still too tough for soloing.