Estimate 3-4 hours if you've never played the game before and are figuring it out as you play the campaign.
The quest instructions are a bit vague at times so estimate a good share of that just trying to find the Very Special Square on the map that you need to step on before the campaign progresses.

after getting the re-inforcements you have to sail the ships around till you can land near the fire shard in the dead lands near the black wall, then the landed units just have to walk to the shard then the wall drops, the other units that were held back by the wall will now be able to join them and all units can then travel down to the dragon army and attack it, when you kill the dragon army you finish the campaign.

harpo