Usually, not very accurate. Video games have to be fun and fair for people to enjoy them, two things that real war is not.

Specifically, with first person shooters, the main character is some kind of Rambo type who single handedly turns the tide of battle. Call of Duty tried to make it a little more accurate by putting lots of other soldiers on the battlefield and not letting you carry 17 guns at a time, but you were still very much the main hero and everything relied on you. Multiplayer had even less to do with real war, since you'd just be running around haphazardly shooting at each other. Even with games where every player had an established role and you had to work together to be effective (Red Orchestra) the usual way of things was a bunch of one-man armies as opposed to even the smallest kind of organized unit.

Now, when you get to the RTS level, things get a little better. Even though you have the unprecedented luxury of knowing immediately everything that happens on the battlefield, you are still in charge of things and get to order your units around. Success depends on how well your entire army fights, not just one guy. However, at this level, you often see a lot of liberty being taken in making factions play differently from one another, usually by shoehorning them into the tired old "lots of cheap and bad units"/"medium amount of okay units"/"few expensive and strong units" slots in an attempt to be Starcraft. This usually means that any semblance of historical accuracy will be ignored in favour of making units that conform to the faction's theme as opposed to how they worked in real life. Either that, or the developers get lazy and makes every faction identical and every tank/infantry squad/cannon is just a repainted version of the same one everyone else gets.