One of the most valuable tools in regards to the treatment of sick fish is the hospital tank, yet it is the most often overlooked and just flat out ignored. Treatment in a planted tank, or more specifically a tank with ANY substrate is problematic and seldom successful. A bare bottom hospital tank insures that your fish is getting the proper medication and it is not being wasted or diluted. It also removes the sick fish from stress caused by other fish. You don't treat a child by giving him medication and then letting him play around with other children.
In regards to all the suggestions given, unfortunately that is the way it always is and will be on a forum such as this. There are many good intentioned people here, but that doesn't always equate to the most sound advice, and when you start following the advice of several different people, or trying everything that everyone suggests, well before you know it you have a bigger problem.
As to your questions, a fish can go a longer time than you might suspect without eating, but this largely depends on size. The smaller the fish, the more likely a prolonged hunger strike will cause permanent problems. However, Fish that are, or have been, subjected to medication often stop eating as a result, and it may take a few days for their appetite to return.
As to how long, there is no one single answer, better pictures would help, can I suggest first uploading your pictures to something like photobucket or flikr, so they can be presented properly oriented? My guess for the moment would be once placed into a hospital tank with dim lighting, give them a week. Monitor closely for any changes in behavior.
I hesitate to answer your last question, because I fear given this information that you will not be able to resist to start medicating again, much sooner than I am suggesting. But since you asked, and based on the information I have seen so far, my best guess would be pure metrozindole, not some diluted or shotgun medication that you will find at an LFS, but 100% pure. You can get this at angels plus or Jehmco.