In the other campaign "rise of all goblins" you can spend 3 exp right off from the start, and this takes alot of time in my opinion.
That's only if you skip the tutorial. If you're learning the game, you wouldn't want to skip the tutorial. It walks you through the first mission where you actually EARN the XP that is just given to you if you skip it.
You could easily get through the tutorial mission in the 3 hour window.