It never really ends. Basically the game is a series of fights, no exploration or anything. You level up your weapons and character during these fights until you get to the end of the bloodline (20-30 min as the guy above said). Then you face the God King, who promptly kills you and the game resets with the next bloodline, however your improved stats carry over and the battles to get to the God King get tougher. Eventually you will get powerful enough to defeat the God King. When you do he comes back more powerful (I think, I haven't actually made it that far yet) and the process repeats.
They continue to update the game with additional weapons/armor/items to level up and in the new year they are adding multiplayer, more weapons/armor/items, more enemies, and actual additional areas to fight through.
Long story short, the game is as long as you want it to be.