To be honest, I'm not sure how long they'll last, as I've never frozen my grains before -- not for any particular reason, except I guess that I use them up quickly enough that it has never crossed my mind...
I do wonder, however, about the effect that freezing will have on them. I know that freezing certain items (for example, fresh fruit) is common practice when they are destined for secondary, in part to reduce any resident bacteria, but also to break up the cell walls, making it easier to incorporate into the beer. Since dried grains retain a small percentage of moisture, I wonder if that cell wall breakage would occur with the grain as well, and if so, whether it would affect the mash (for better or for worse). Perhaps the moisture level is too low for that to happen, but now I'm curious. Maybe someone with a bit more information on the subject could weigh in here?