How many hours a day can I provide care before I'm considered "live-in"?

Do you have a legal residence someplace else?
What is the address on you Drivers license or State ID?
What address is on your vehicle registration?
What address is on your Voters registration card?
Do you have utilities in your name that you pay for at another address?
Do you have a caregiver contract that indicates your duties, the number of hours and what those hours are in writing?
I would think legally if you have everything registered at another address, you pay bills for a residence and pay rent or mortgage at another address you would not be considered "living" at the address where you are employed.
If you do not have a caregiver contract spelling out your duties, the number of hours that you will work you might want to have one drawn up. Also if you are getting paid (and you should be) that should be indicated in the contract as well. (X number of hours per day, for $XX.xxx per hour. And indicate the repositioning that needs to be done very few hours.)
You might want to talk to your partner about the possibility of an Alternating Pressure mattress so that repositioning is not so much of an issue. The alternating pressure mattresses will inflate, deflate and change the pressure in channels that run through the mattress so that there is always movement relieving pressure all over the body.

Not sure about section 8 but I know someone who was on a housing grant and no one was allowed in her apt before 10am, I think, and not after 10pm, that I am sure of.

If she is getting housing, is she getting Medicaid for health? If so I would check out Medicaid "in home" to see if she qualifies. She may only get 4 hours a day or a few hours a week maybe more, but whatever she gets means you don't need to be there. Aides can do most of the heavy work leaving you with the lighter tasks. You may find there are resources out there she can take advantage of. She can apply for in home help even if not getting Medicaid for health.

I just read about Hereditary Spastic Paraplegia. This is not curable and can cause all kinds of problems. You friend may not be able to stay on her own forever. She may need Skilled Nursing eventually. Please be aware that no one can care for someone 24/7. As said, you must be exhausted. You need to find help for her.

She cannot be kicked out of she has 24/7 care prescribed. You may be required to get certified as a CNA if you want to provide that care. Are you exhausted?

Your profile says your partner is "is 41 years old, living at home with mobility problems, osteoporosis, and urinary tract infection."

Is your partner on SSDI? Medicaid? Can your partner get a letter of diagnosis and prescribed repositioning from the doctor to show the housing office? More info would be helpful.