Been through cancer twice now with very dear relatives. You can't predict how long you have to prepare. My advice is to start now.
Your thoughts will be dominated by your dad, and rightfully so. But I can't stress it enough how important it is to take care of yourself, too. Whatever helps you prepare, whatever helps you grieve, whatever helps you accept -- you need to do that for yourself.
Your own peace of mind is of utmost importance.
When it's time, your dad will be gone but he will be at peace. Putting yours off and having it slap you in the face when he's gone makes death -- such an inevitability of life -- even harder to deal with. It doesn't have to be.
Cherish the moments you have left, make him happy, do whatever he wants, and appreciate all the time you can spend with him.
But do so with the knowledge that he won't always be there, and the peace of mind that you can accept that. Good luck, and my thoughts go out to you and yours, Pall.