How many hours of gameplay would it be to do all the single player content with it in expert mode
This will be a huge guestimate from my side as I've only played Normal and Hard but here's the breakdown.
There's 4 grand prix with 4 races in each. A grand prix is about 15 minutes. So all 4 is an hour. BUT you're talking Expert which is quite challenging, so not everything will be 1st try, atleast for me, as it was for the other difficulties. Could image it would take 3-4 hours to do all Grand Prix on Expert.
So that's 4 hours but there's achievements connected to this mode. Every driver has 4 cars. You need to win a Grand Prix with each car for each driver to unlock a customization option and achievements. So that's 40 Grand Prix and keeping in mind that you get better as you play I'd say that this grind is around 30 hours.
On top of that there's single race where you can pick Arcade, Cops and Robbers and Chase the Bomb or whatever its called. I've only tried Arcade Mode, and there's achievements connected to all of these, but you choose a track and a car and go. So that's 16 races on Expert, so 3-4 hours just like Grand Prix. On top of that you can play in mirror mode too here so that's another 3-4 hours. Don't know about Cops and Robbers or Chase the Bomb as I haven't tried those but I could imagine it's also all tracks and mirrored. No idea wow difficult it is? add an extra 10 hours?
So we're up to 30 hours for the Grand Prix + 18 hours for this single race stuff = 48 hours.
Finally there's Time Trial. There's a staff ghost to beat on each track. I also think this has mirror mode. I only tried the first track and after 10 laps I was still 1.5 seconds behind. This could take quite a while. 30 minutes per staff ghost on average times 32 tracks = 16 hours.
So a combined 60-70 hours to unlock everything on Expert?