A weak stressed out fish can be lost to ick. from my experience though, a healthy fish will just go through a cycle of ick and that will be the last you'll see of it on that fish. The weaker the fish, the longer it will take to pass. If the fish is healthy and eating well it should pass in a short time unless it's an ick magnet like a powder blue tang or something. I also find tank temp makes a difference. 80 degrees is the magic number for me!
I've found that they get over it faster at this warmer temp rather than like 76 degrees or something like that, so I just keep it at 80 all the time.