For me, usually around half an hour for a field battle and an hour or more for a seige.

It largely depends on my forces and the enemy, i'm terrible for going over the battleground inch by inch in full zoom looking for every ridge and depression, to give my troops the edge.

With a siege much of my time is spent moving troops into position for the final assault once the walls are breached and the enemy has fallen back, very rarely in my games do the enemy try to hold the walls after i make a breach, they always seem to pull back to the square. So much of my time is sending forces to flank and split their own defence.