Bought the game 1 week after it came out i think, been running B spec almost non stop for a week or so and will hit lvl 40 next week thursday. Im on my xbox while my bob grinds 24h le mans.
Did no grinding whatsoever, I'm stuck on lvl 24 for a while now since I can't be bothered to do the extremely boring 10 lap races in the extreme events. So for me it will take ages to reach lvl 40, as i mainly play online now. Really bored with the single player events, especially those with 10+ laps.
As for B-Spec, I'm starting start to love it Didn't like it at first but now I love to seat a lvl 1 b-spec driver in a 900HP FR supercar and watch him drift like a maniac.