I think they've said about 20-25% of the full game. Recently Swen said in an interview that there would be 5 large maps. The alpha has about 66-75% of one of those maps.
Being able to get to level 10 so soon might seem odd, but my guess is that the level curve hasn't been adjusted yet, in the final we may not be at that high a level from finishing the alpha content. XP required to reach the next level also tends to increase steeply the higher level you get.
EDIT: If I remember right, during the kickstarter, they mentioned level 20 as about the intended max level for the player. I don't know what their final plans are going to be for that, though.
