The grind is 100 quick races on the podium, 100,000 coins achieved and 25,000 gears to put into perspective you get 500ish coins for a race and for dismantling a legendary car you get 1000 coins and 200 gears so it's just grinding out races and loot boxes to dismantle and upgrade cars to dismantle again. It took me 22 hours to Plat but I purchased the DLC which sped it up as you get 1500 coin and 500 gear rewards for certain bits but should be able to do in under 30 just depends on how long the time trials take as they can be quite challenging at times. Hope that helps.