How long does a Reign last in Dragonfire?

A Reign lasts about 8 weeks. That's the length of a full season: you start a new map, redevelop your stronghold, and chase the campaign objectives before time runs out.

Inside those 8 weeks, the "Heirs of the Dragon" campaign spans ~40 days (11 chapters plus an epilogue). It sets one hard objective: reach Stronghold Tier 10 before the Reign ends to be able to claim the rewards.

A note on pacing: don't dawdle early in the season. Since everything is re-grinded, the faster you raise your stronghold and your House Level, the sooner you reactivate your permanent bonuses (Heirlooms) and the longer you enjoy your full strength.