When to transition someone to assisted living depends on you and your sister.
Although, you've cared for her for six years, it sounds like you're both getting exhausted.
The answer to your question then depends on how often your mother has those nights when she gets up frequently versus the nights she sleeps.
Also, the adult day care center can provide you with some guidance since they see and interact with her, daily.
Care Options for Your Mom
You and your sister can take turns getting up to help your mom when she needs help.
If she doesn't need help each time she gets up, you can catch up on your rest.
You can hire in-home help one night a week.
This will take the pressure off of you while not costing as much. I suggest breaking up the week and hiring help on Wednesday if you work Monday through Friday. Alternating each week to two nights a week (Sunday and Wednesday or Tuesday and Thursday) will still cost you less than a nursing home.
You can try out assisted living for a week while you take a respite.
This will give you time to catch up on your rest and even feel re-energized. You may even consider a board and care for a more home-like touch with only five or six residents.
Trying these options one at a time will ease the pain (guilt) both of you may feel as long-term caregivers for your mom. It will also help you to ease the transition into long-term care for your mom.
If you wait too long, the care facility may not accept her. This is a possibility.
Bottom Line: Your mom may surprise you and thrive in assisted living.