Alright, someone else will be along who knows this better, but here goes:
Combat: the attacker rolls dice for his units, and whatever scores a hit removes a unit from the board. ex: you attack with 8 infantry, 5 armor. You roll 8 dice for the infantry and 5 dice for the armor. You roll a 1 with the 8 infantry dice, scoring 1 hit. You roll 2 2s and a 3 with the armor, scoring 3 more hits. The defender has to remove 4 units from the board, but those 4 get to shoot back before dying.
Dead defending units always retaliate, except naval units that get hit by subs.
Game length: my games tend to run between 4-8 hours, although I'm pretty novice. Expert games tend to take up entire weekends.