What I do is ... shards.
Okay that is not a good explenation and maybe a lot of my genes have to do with (yes, I meantioned being German at least three times ... so here comes my fourth time ... HURGH!).
I know about how long I need per page to draw, or how much it does take me to write a script. Now this isn't too far away from writing. I know since I used to write a FRIGGIN LOT for university. My master thesis alone has got over 150 pages worth of words with content (or content with words? What ever).
Back then I KNEW how much I had to write per day. (6 Pages by mean, but sometimes it were only 2. On others I had a flow and could write to infinity ... not really, I am not Chuck Norris.)
I'd just try to make a plan. Write out what you think you'll be able to do. If it is too much or if you still have time for more you can always adjust.
Go by "weekly" and then calculate the hours per day.