From 0%.
All gold in B License: 45 minutes (Getting a Miata LS)
Getting A, IB, IA Licenses: 45 minutes
Slapping SSs on the Miata and plowing through roadster endurance: 1 hour 15 minutes
Assuming you got a Miata... (If you get an F-1, it's over)
Buy Mercedes SLK 230, slap a Turbo 3 and some other mods on. Sell both Miata's if you like to add more.
Beat Beginner Silver Arrow: 20 minutes
Sell the SLK 230 you just won and complete the mods to the Merc.
Beat Amateur Silver Arrow: 40 minutes
You now have a CLK Touring Car and 60k.
Plow through Beginner GT World Championship: 1 hour
Use the saved game trick to get the Skyline GT-R: 15 minutes
Now max out the Touring Car with a NA 3.
You now have a Touring Car can plow through any Beginner and Amateur race it can enter and a GT-R plus 100+k credits.
If you want, spend the remaining 2 hours do two of the shorter endurance races for more credits and 2 more shots at F-1s.