The time the 171 takes varies depending on the time of day, but it seems slower than the 172 to get to Elephant & Castle. I get the 172 every day now rather than the train, which gets me to Covent Garden in about 50 minutes at rush hour - only about 10-15 minutes longer than when I would take the train (and I get a seat all the way there). I could also get the 171 as they both go to Aldwych, but have found that the 172 is much quicker as it doesn't go through Peckham and Camberwell.