I don't use a lot of cake flour but, what I do use is always stored securely in air-tight containers. I'm pretty sure some of my cake flour has been around a couple of years. I have never noticed anything "off" about it when I use it.
I'd say as long as there are no obvious "creatures" living in the flour and it's not clumped, which would be an indication of accumulated moisture, go ahead and use the flour up. And, I think, lulu's suggestion of using it in highly-flavored cakes would be a good idea, too, if you want to be extra cautious as to any stale taste.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!