Yesterday, I got a package from Fender. Three packs of .012 phosphor bronze strings. I haven't tried them out yet; I'm still rocking the Earthwood .013s, and as I got two packs, it'll be a while before I'm past them. But I will use them, I'm sure. I'll re-review when I do. It's funny that I used to have no spare packs and now I have four, five if I want to reuse the .011s that broke in the first place.
In my experience, the wound G is the first string to pop, and I figure that's the time to switch, since the D will come along eventually, and if you switch, you don't have to worry so much about your other strings popping on you. But that's me. When do you know you have to switch?