I began interviewing in Sept. 2014 and it dragged on until the beginning of 2015 due to a company re-org. I had 1 phone interview, 3 in-person interviews in Schaumburg, and 1 in Milwaukee. I sent thank-you letters to everyone I met and included key items we discussed during the interview and how I can help, even sending helpful HR articles to help with HR issues currently being faced with. I followed up with the recruiter more than 3 times. Despite my travel efforts, time for the intense interviewing, and multiple attempts to find out the status with recruiter, nobody told me the final verdict of my candidate status. I think somebody could have at least said, "Sorry, no-thanks" then to simply not reply.
Recommendation: Set clear boundaries on your end of how long you're willing to go to wait around to complete the interviewing process.