If I enter the US on a K-1 / I-129F Visa, to marry my fiancé - do I have to leave after the 90 days granted?
After marriage the I-485 visa will also be applied for, do I need to leave until the I-485 is granted?
You are admitted for 90 days for K-1. Once you file I-485, you can stay in the US for as long as your I-485 is pending. (This is true even if you file I-485 after the 90 days of K-1 status is over.)
You cannot leave the US while your I-485 is pending, unless you've been granted Advance Parole before you leave, or you would automatically abandon your I-485. You can apply for Advance Parole with I-131 for free together with your I-485, and it will come in a few months.