From the horse's mouth...

Per Reuters, via TPM:
"U.S. presidential candidate Barack Obama talks about 16 months. That, we think, would be the right timeframe for a withdrawal, with the possibility of slight changes... Whoever is thinking about the shorter term is closer to reality. Artificially extending the stay of U.S. troops would cause problems.... The Americans have found it difficult to agree on a concrete timetable for the exit because it seems like an admission of defeat to them. But it isn't," [Iraqi PM Nuri al-] Maliki told Der Spiegel.