It takes about 1 week to build in 80% of my games. The difficulty is always set to maximum. There's only one issue with the monsters at the mine. If I set it to the maximum, everything slows down for exactly one week, just like for everyone else. And the reason I said so directly that Necropolis is the best on a random map in Heroes of Might and Magic is because they have more things they can use to their advantage. That's all. Honestly, half of the mines can be cleared even without archers. Yes, with some losses, but minimal. The only thing that annoys me is when archers defend the mine at the beginning of the game. That's a real pain...