There are a total of 52 events in the career.
It takes 10-15 mins to dominate an event, maybe 20 if you llike me, have trouble with this games driving physics..
In my quick calculation that should be 12-14 hours worth of your life.
But it's real easy, to just win around 75% for Domination of an event, especially if you get good scores.
At the end you'll face off Ryo..
Like some of the other guides said, a good choice is the Porsche Stage 3+ Widebody kit, to get you there. Use it for Grip etc.
I used Supra for Speed, with Stage 2 tuning+Widebody kit.
I had the Charger for Drag, fully tuned.
And focused on winning Aki's RX-7 for Drifting.
Used the Corolla, full engine power+stock wheels slow but easy.
So save your money to buy Drag-power and Porsche 911, is a good advice
Good luck and frustration to you
PS. By the time I wrote this, I was on last Showdown and just Ryo left after.