I replied to this way back on about page 2, but this thread has stayed with me.
With my 'safeguarding' head on, and to reply directly to the OP.... There's a lot of anecdotal advice on here, based on PPs experiences/knowledge of their kids. (including my post). And YOU know your child, so do listen to your own inner voice.
Given your updates, I suspect you think you have no choice but to leave your child, and in a sense you're looking for 'permission' to do so via this thread. If this is right, I'd say that's not a sensible thing to do in RL, and if you DO have legitimate concerns (because you HAVE to be elsewhere and you're worrying about it) then listen to your inner voice, and if you're worried (which you clearly are) don't do it.
In safeguarding terms, you should know that there's no minimum age to leave a child. However, if you do leave a child alone and something goes wrong, the consequences for the parent stack up hard and fast depending on things like the child's age and abilities. So if you have a baby and go off out to the pub for the evening and something goes wrong enough for people in authority to get involved, you should expect the consequences on you to be severe. The older and more able your child gets, the fewer the consequences and the lighter the judgement.
But, you've said your child has MH issues.
So, imagine you knowingly leave your (say) 12 year old home alone for a set time. Not so horrific. But then something goes wrong. Let's - hypothetically - say a fire. Could be caused by child doing something silly with the toaster/playing with matches/experimenting with cigarettes - whatever. Could be something totally random - a fault with the electrics which just happened to fail while child was there and you weren't. Doesn't matter how. Just means people who have to report to social services under their safeguarding policies are now swarming round your home and your child.
They would make that report knowing:
You've knowingly left this child alone.
You know this child has MH issues.
There - clearly (if things have gone wrong) isn't enough support in place.
I'm not saying this to get at you or be mean, OP. And I do stand by - you know your child.
But I would urge you to appreciate how things might come back on you if things go wrong. Please, please, please feel free to read and ignore if I'm off the mark. I am just a random off the internet. And I do - honestly - understand how hard it can be to be a single mother with responsibilities outside the home.
And to caroldecker you have a horrible username, and a horrible outlook. I pity your DC If a 12 yo NT child can't do these things you have failed as a parent. Several days is fine if they are happy with it.
I don't want you as my mummy!