I'd recommend starting to think about college applications in summer, but never finishing them! So many things changed for me during my senior year, that my apps would not have been the same if I had finished them in the summer. This was the same for most of my friends as well, we all discovered what we wanted during our senior year, even without knowing we still had to discover it. In many ways the application process, for me at least, did not ruin my senior year, but made it!