I bought the pre-order today, about 10 hours ago...have maybe 10 games played already (I am going to freaking LOVE this thing when it ships!!!) and against the Hard AI, I usually win in about 15-20 minutes now. That's playing with Regulus and the Rook. With Sedna, it took almost 45 minutes (not really a fan of the General style of Demigod), and with Oak about 30. Queen of Thorns took forever, and even with so little playtime I can see why more people aren't fans of her. I haven't tried Unclean Beast yet but I'm betting I'll love him.
My usual play againts the AI- and it almost never fails -is to pick a side, either left or right, and just wear down the towers on that side while beating back enemy Demigod and troop attacks on my defenses in the middle, and on the side that I am not attacking. Sooner rather than later, you end up capturing one of the two portals and at that point, it's all over for the AI. I made the mistake though of playing in one 2v2 game this afternoon and, after just 3 Ai matches, that was a mistake. I cost my teammate badly 
I always have a teleport scroll on me, and at least one heal potion. Everything else is geared to recharging mana and health for me.
I have to admit...ever since reading the first previews of this game ages ago, I have been in love with the idea. I'm going to absolutely binge on it when it comes out. Demigod, even in Beta, rocks.