As a PS4 player, I recently shifted to the Xbox and got Forza 5 and horizon - great games btw… Anyway, I can tell that Forza 5 is a really really long game to beat 100% career and I was just wondering how longer or shorter is Forza 6 compare too 5. 
Kinda long with multiple endurance races.
I believe there are 8 non-expansion endurance races that take 2-3 hours a piece to complete.
Each expansion should take 10-20 hours a piece to complete.
The base career mode in FM6 has to be beat 6 times over to get 100% and gets repetitive quickly. The races are short but it seems to be never ending with zero reward at the end outside of an achievement.
Its a little shorter, the last half very boring.
The career is a fulfilling mix - far more diverse than F5. Don’t worry about other folks’ opinions. Get both expansions and enjoy!
I must admit though I find myself mostly hot lapping or having a 1-on-1 with the AI.