How long does it take ich to spread and kill? Ive never had a fish with a disease like ich so im just wondering

Ich is a parasite that is always present in fish tanks. Ich only attacks the fish when they (fish) get stressed and their slime coats thin. The ich then burrow under the fish's scales and will usually kill the fish in a week or so depending on how badly infected the fish is. Ich can be killed by raising the temperature and medicating the tank. Aquarium salt also helps. By raising the temperature, the ich's life cycle speeds up, so the medicine will kill the numerous swimming larvae that are produced by the temperature induced spawning of the parasite. Changing the waterm keeping it at a high temperature, and adding salt and/or medication will eliminate ich at least 60% of the time :)