How do you know when it's no longer safe for your parent with dementia to live alone?

My Dad, was diagnosed with mild dementia in january 2012. Since then it seems there are more bad days, but overall he's still pretty independent. He still lives alone, though I'm there everyday from about 4 in the afternoon and I stay til he falls asleep, and if it's a "bad" night, I stay all night. He's been sundowning, but once he's sleeping he usually sleeps through the night. He doesn't know me as his daughter anymore, but knows that I am karen and that he can call me anytime, which he does.actually sometimes he thinks there is 2 of me. It seems with the shorter days now, it's making the sundowning worse, some days he gets confused etc, as early as 3 in the afternoon, but has not tried to wander. At what point should he not be left alone anymore?