Pretty simple question but I think some recommendations would make my chains a bit long-lasting. So when should you do something to your salted chains? When the chains already look rusty, can you use water to clean the dirt out and then oil as a lubricant, or is it better to throw rusty chains into the dustbin? So how to maintain chains in good condition during winter? Oil every day, what about water? May water do even more damage to the chains if the metal has become very porous?
You can use water to remove the salt if it's bad enough, but immediately wipe off the water and generously re-lubricate the chain. You do not want to let the chain sit around with the salty water on it, as it will readily rust. So if you use water, just make sure the chain is completely dry afterwards, then re-oil it.
Metal doesn't really get porous, but AFAIK rust kind of does-- if the rust is that bad, you may want to consider cleaning the rust off with a toothbrush or getting a new chain altogether.
Oiling often in winter is a must. Some people use hot wax instead, but depending on your situation, that might be overkill. You may not need to oil it every day (if it still has plenty of oil visible on it, I don't bother with my chain).
I wouldn't say that you need to throw your chain or cassette away if it gets that rusty. I do some work restoring bikes and have found that you should soak stuff in Liquid Wrench maybe 20 minutes to an hour. Then take steel wool and scrape the rust off (fine steel wool works best). Works wonders. Then dry and lubricate with oil.