It is totally possible to become a writer with a mental illness, even Schizoaffective Disorder. I'd suggest that you may have greater self-awareness and coping strategies as a result.
Whatever works for you. But start with a manageable and reproducible target. Even if it's one hour per day. Start there and grow to suit your situation. If I had to pick an number I'd say aim for 2 hours per day. You can write 50,000 words in a month - that matches NaNoWriMo target and is the length of The Great Gatsby.
Most beginner writers aim for 2 blocks of 2 hours (that they fit in before and after school/work). Interestingly many professional writers I've spoken to also write in blocks of 2 hours. Very few write non-stop for 6-8 hours a day. I suspect that those ones dictate to a transcriber or dictation app. Dictation is a skill I wouldn't recommend learning just yet.
Like playing a musical instrument writing is a skill. You cannot begin writing at 8 hours per day. Instead build up to it based on what works for you. Professional classical musicians practice from 1 to 8 hours per day, but research suggests results diminish after 2 hours.
Mindful writing is required. Actually producing words, not editing - that is a separate effort.
How to stay motivated
Log your work output and effort
Count your daily word output and log it somewhere. On a spreadsheet, in an exercise book, on a whiteboard (take a photo of it from time to time). As you produce words the "run-rate" and total word count will motivate you.
What you write at this stage doesn't matter. 800 words on the outline or plot is still 800 words. Just classify it as outline, plot, Chapter X.
I find it useful to also log the time it took.
You may like to add up to 10 words to describe the session (feelings, thoughts, whatever but keep it brief)
The First Draft is sacred
Writing is about starting. After that writing is about finishing. Finish the draft. It doesn't matter if it is good, you're not going to show that first draft to anyone.
Get the words out. If the writing is ugly or ineloquent, keep going. Trust that you will fix it in the next draft. If you notice a minor plot problem make a note for later (e.g. how does our hero fly this plane if earlier I wrote he has epilepsy and never learned to drive?)
Major plot or character problems may require you to retrace your steps by going back and changing things. I would suggest making a note about the problem and proceeding if possible as if you'd fixed it earlier.
The goal is to finish the draft. It is like climbing Everest. Once you are in the danger zone you have 2 options - reach the summit or return to base camp.
Don't judge your work until you finish this draft
Our inner editor/reader/critic is the main reason most people do not write more or finish much.
Separate Editing from Writing
Don't edit while you are writing and don't write while you are editing.
After you write X words or Y minutes/hours take a short break before reviewing the words. Ideally don't revise the work until you have the first draft completed. If you must review I suggest reviewing the previous day's work or even the work from last week.
Avoid the trap to you tweak as you write.
After you finish your first story (short- or novel), start the next one immediately during your next writing session. Edit the first story in your editing session. So if you write for 2x2hrs each day you can either make the second 2hr block an editing session or you can fit a third 2hr editing session in. See how you feel.
Keep reminding yourself that you will fix the writing and plot problems in the next draft.
Plan your writing
Create an outline - it may or may not be "canon" for your story. I find my characters go off on their own.
Break the project down to chunks. You don't need to plan those chunks down to daily goals. They should be specific and they should be achievable in a week.
Every writing session has an outcome. It can be either a word count or a story point. Tell yourself what part of the story you are telling this session.
It's okay to start with short stories and grow from there.
If you get distracted after a chapter or two spend some time writing short stories that are finished in one writing session. Then work on longer short stories that take a few sessions to finish.
Build up to stories that take a week to finish.
Finally you'll be ready to move to stories that take a month to a year to finish a draft.
Practice your editing process on the short stories if you like.
Have a Workspace
I write on my laptop in a variety of places but I have a special place that I churn out words.
The familiarity reinforces your brain that this is writing space and time.
Put a written goal visible at every writing session.
Use either a post-it note or a big sign above your screen, but put up that goal.
Today I will write "The hero arrives at the camp"
- Write at the same time every day.
- Write for the same amount of time. This is beginning advice, some professional writers keep writing until they get their word count every day. Journalists do this too.
- Start writing at the beginning of the writing session. No Tea or Coffee making, outline reviewing, twitter/email/stackexchange checking. 9:00am start producing words.
Maintain a Sparks file
A file (or collection of files) to collect ideas (i.e. Sparks). Periodically review to refocus and generate new tasks. -- Fritz Freiheit on Sparks
This helps with motivation but is really about writing process.
I have a text file in my writing directory called _Sparks.txt (so it sorts to the top). I have it open in my editor but minimised.
If I get a great idea while writing my current goal, I quickly type the pertinent points out in the Sparks file and save it for later. If later I can't tell what was so fascinating about the idea then it was just a distraction. Normally I still like the idea when I return to it.
My sparks range from single lines e.g.
* I am a minor league mansplainer
to 500 words of plot or setup or even a picture I found online (I save picture inspiration to pinterest.com)
I count words I add to the spark file in a separate column on my daily log.
When I want to start a new project I open the Sparks file and pick a project. I create the folder for the project in my writing directory, I cut the spark note out and put it in the project directory, along with other inspiration and log files.
A note on Revisions
I print out my first draft double spaced with 1" margins.
I edit with a pen and hand write the changes.
My second draft is then written by retyping the entire thing incorporating my notes and changes - no cutting and pasting. That is the second draft.
Rinse and repeat.
Nobody ever gets to see my first draft or any draft I haven't finished because...
The First Draft is Sacred!