I replaced the ignition switch first, since in my Haynes book, it is what is directly connected to the fuse. The car works fine for a day then blows the fuse again.
If you look at the wiring diagram for that fuse and your manual see it probably hooks to a lot of things. Find out what else the fuse is connected to. 50 amps is a pretty big fuse so that might just be the main fuse for the entire car. If that's the case you need to look at your wiring diagram and see what electrical circuits get energized when the ignition switch is just initially turned on. Blowing such a large fuse means you probably have a short to ground and you'll need to trace the wires (both in the diagram and physically) and you'll probably find a charred wire some place. Unless it's something obvious that you can see quickly you may just be better off taking it to an automotive electrician to find the short.
Your starter is starting to go bad and is drawing too much current. Pull the starter out and take it your your FLAPS to get tested. Then replace the starter.