Jackson, a lot of folks would say that if you can see rust at all, it is time to condemn the cables. Double-check by running a cheap white rag up and down each cable. If it shreds even once, from just one meathook? There are internal failures that you can't see, and it is time to condemn the rigging.
The modern rule of thumb seems to be replacing stainless rigging every 10 years (tropics, salt,heavy use) to 15 years (northern seasonal use) and at 20 years, don't even ask.
If you've got so much rust that you need a wire brush...don't even ask. Cable is way cheaper than a new mast, medical bills, and a tow home. At least get a pro to look at it.