How long will my hair dye last?

My hair is fine and dark blond naturally. Today, I dyed it with L'Oreal Superior Preference in the 2nd lightest shade. It's fairly light and a bit reddish (not too bad but not really what I was going for.) I was just wondering how long it's going to take to fade out. I use John Frieda's Sheer Blonde shampoo and Body Envy conditioner. Any guess you have would be great. Any tips you have are also appreciated. Thanks!