1. does blonde dye wash out over time?
A) No. While the tone might change if you went from a darker color to blonde, you actually stripped your hair of the pigment, rather then depositing color with a dye.
2. Save it without cutting?
A) have you tried conditioning treatments? If your hair is really damaged beyond repair there is nothing you can do to save it. I know you don't want to cut it off but you can get a trim every month or so to promote healthy hair growth.
How often were you coloring the whole thing? Were you doing it at home? I'm going to assume you were. if you're dying your hair blonde you really only need to do the roots, the rest of your hair is already blonde. Sometimes a blonde color will get a little "dingy" in which case I would recommend a toner. If you don't have the money or time for that, try doing just your roots and when thats fully processed take the rest of the dye and add shampoo. Use this mixture all over your hair, this will deposite a bit of color all over while not being as harsh as straight application. Rinse it out as usual and do the conditioner.
Hope that all helps