My (solo) experience last night was about an hour. This was new stuff, so not exactly sure where I was going or how I was going to get there. I strongly suspect that it will reduce to much shorter times once players are more familiar with how the boarding section runs and how the underwater section maps are laid out. In terms of length, the underwater section is not quite as long as a dungeon; it's trash packs, a couple of bosses, and maneuvering around some environmental hazards. Boarding section depends on how long it takes you to sink or drive off the attackers.