There are TWO dungeons. the Daily one and the Weekly one.
You get into the Daily dungeon by completing Daily challenges, you get a "normal" Frontier Key soul. Open this soul and you get into the Daily Dungeon which last 8 hours, or 3 runs
Successfully completing the Daily dungeon gives you 10 RARE Frontier Key Fragments. 200 fragments gives you a RARE Frontier Key Soul. (Covert fragments in the BOS). This RARE Frontier Key opens the Weekly Dungeon which is available for 24 hours or until you complete 3 runs