1My Ramirez strings are showing wear marks on the silver already and I've only got about 10 hours on them. The silver plating is really thin on guitar strings. I change them when they have significant wear on the windings. By the time the d string is toast, I just change them all.
I think the OP said in her post that she uses D'Addario, in which case shouldn't have much to worry about. When I've used D'Adds the basses seem to last a good long time, before the windings start to dent, and they are inexpensive when is time to change them.
Just out of curiosity Kirolak, how much is a set of Pro Arte where you buy Them? Strings by Mail has them for about $7.00 USD, and they ship worldwide.