BioWare put a whole bunch of hooks into the game, revolving around "events" to track things that players were doing. For example, the developers could track when dialogue was skipped by the player, leading them to discover that 15% of conversations were skipped by the total player population.
Looking at all of these events on the PC and Xbox 360, BioWare found that the average completion time was 33 hours and that around 50% of players imported a save game from the original Mass Effect.
More data can be found in the picture above or at IGN, along with some discussion from Casey Hudson. A whole new mess of data will be created in January when the game is released for the PlayStation 3.