I agree with the others women do have to get it off their chest so they like to just vent and most guys just let the woman talk and basically tune them out. They aren't looking for solutions, they aren't trying to get your sympathy, they aren't trying to fix anything
Just let it out so it isn't all bottled inside to where they want to explode. So just half listen and treat it like she is reading off the grocery list to herself. Then when she is done say something like sounds like nothing changes there does it.