I tried Barb & Necro in the Open Beta Early Access and Open Beta itself to both level 25.
Both were so vastly different in difficulty, I just needed 2 - 3 hours to bring the Necro to level 25 completing all the main campaign in Act 1. In the Server Slam, I did both the Sorc & Druid, Sorc to level 20 and Druid only did up till level 6 first reaching Kyovashad. You can tell it was easier for the Sorc & Necro.
I think fully focusing just on Main Campaign for first character will likely take us at least 10 hours for first playthrough without the Capstone Dungeon. Any characters after that don’t have to do the Campaign anymore and we’re free to just do Altar of Lilith, Dungeons, Side Quests, Renown Rewards, World Boss, etc. so it should be shorter.