6 to 7h for Career any%, + let's say 1h30 for the remaining races & milestones + 5h for challenge series + no-one-knows time for rap sheet completed (at least 5h).

So in the very least, 17h I'd say. Maybe a bit less.

Does career 100% require CS?

Career 100% is only winning all races & doing all milestones.

Game 100% is everything

Game 100% can start anywhere you want as long as your savefile is new.

To be sure of it you should show every menu before you start the run, to prove you haven't done anything on that savefile