There is no guarantee, the more people applies, the harder the cutoff time. 2015 the cutoff was 1:02 under BQ (25493 BQ runners applied, 23546 got in). 2016 the cutoff is 2:28 under BQ (28594 BQ runners applied, 24032 got in), but 2016 is special because it's 120th anniversary so more people were trying to get in.