You'll find a plethora of posts here on disengagement.

I think each situation is different, and how disengagement works is unique for each person/family. I can tell you what it looks like for me. I no longer approach the skids about doing chores/cleaning up. I ask DH very specifically, because if I'm not specific, he has no idea what needs doing, to get the skids to do what I need them to do...for example "DH, SS and SD have no laundry in their hamper but there are clothes and towels all over their rooms. I need you to get them to put the clothes in the hamper so I can wash them". Then it either gets done (by them or him, IDC who) or it doesn't...if it doesn't get done oh well, no clean clothes or towels for them. I no longer take their wants into consideration when I am buying anything. I will get what they NEED (TP, shampoo, etc) but if they want a certain brand of whatever or a food that I normally wouldn't get, then DH needs to do that. I don't transport them unless it is of benefit to ME to do so. I don't try to make conversation with them (they either would give one word replies or give me blank stares...). I rarely go out with them to shop, eat, movies, etc. even if DH is along also because I feel excluded and they have poor manners (I used to try to work on that, but without parental reinforcement, it's a lost cause). I used to try to encourage better eating/drinking habits, but again, if there is no parental reinforcement it's a lost cause and why should I stress myself over it? I almost never offer my opinion on the skids anymore...DH used to ask me then get mad when I gave him an opinion because I was "negative".

As far as when, well, only you can decide that. I didn't just all at once stop doing everything. It was a gradual thing. I had to pull back to maintain my sanity and keep stress out of my head. There was no announcement made either, I just started doing what I needed to do to keep ME happy.

You should have been sending this kid back to his mother EOW and refused to 'babysit' him during your DH's 50%. He's not your kid and not your responsibility.

If your DH wants to divorce you for refusing to play 'Mommy' 24/7 to his obnoxious 12yr, so be it.

When BM dumped the kid, your DH should have modified the custody order and went after CS. You could have used it to send kid to day camp all summer. Disengaging from the kid isn't going to stop him from breaking and taking your things... that's only something his father can do. It's called discipline.

Your son and your new baby deserve better. So do you. Time to let Mom and Dad parent what they created.

Disengaging is different for everyone. I also have a SS12, and disengaging to me is basically nothing past civility- as far as face to face communication with him. As far as the other stuff, I do NOT drive him to/from soccer, I do NOT do his laundry, no cleaning up after him (even though the bathroom he uses gets disgusting!). He tries to talk to me, I simply answer with "Mmhmm" or "oh really". Most nights after BS2 goes to bed, I simply "work" in my bedroom. DH gets pissed when I go and hide, but I have been disrespected enough by the brat. I owe him nothing and do NOT need to earn his respect. However he should respect me in my home. Period.

Your level of disengagement really depends on your own unique step situation

With that said, generally my rule with how engaged to be with skids is really dictated by them

I have two adult SD's, I'm close to one with a good relationship, and as disengaged as I can get away with, with the other one

The YSD has made it clear I'm family to her, she values me in her life, appreciates all I've done and continue to do. She wants her SM in her life, and so I'm the best most involved SM as I can be to her as this is not only what she wants from me, but I'm comfortable and happy being a part of that. It's rewarding for both of us.

For my OSD, she has made it clear she despises me, I hold zero value in her life, she does not consider me family in any way and even to acknowledge me as her SM (which I am) is something she would never admit to

So, I have followed her lead just like YSD's lead. After years of trying with OSD, hoping to win her over, thinking she would grow up and change, I finally realized the best thing to do in our situation was respect the messages she sent to me loud and clear to have nothing to do with her

So I'm not involved in her life any longer. I know she doesn't consider me to be family in any way, just the wife of her father and I mean nothing to her. So I don't try to be family to her, certainly not a step-mother in any way, I'm pleasant and polite when I see her, but that's it

I also know she is very jealous and insecure about my relationship with DH, and that she plays very spiteful, sly games aimed at hurting and humiliating me. These games include her 'cycles' with me, going for a few years behaving in a hostile, nasty and belittling attitude to me, and then when she sees she has gained nothing and no reaction from me, switches to apparent acceptable and is even somewhat nice. I used to fall for that and reengage with her, only to be then treated like crap again, so now just all the more reason for me to stay as involved as I can from her

Another main reason people disengage from their skids, is to manage their own stress level. Sometimes it's better just to stay out of everything to do with your skid, and avoid problems that would otherwise arise if you were part of something that is only going to lead to hurt, anger and frustration.

I used to fall over backwards trying to please OSD, trying to get to know her and find common hobbies and likes, bought her many thoughtful gifts/did very thoughtful things for her, paid tons of attention to her, couldn't be warmer or more welcoming, only to have her put down the things I did, refuse to even so much as say hello to me, or worse, accuse me of butting in where I had no business - and if I then was uninvolved as now, I'm accused of not caring and having never done anything for her. It's a no win. So, I make it a win for me by just not giving her anything at all to use against me any longer. I simply smile and wave, I think of her as a neighbor's adult daughter, one who has problems and is best avoided for my own safety and sanity, and treat her as such.

How old is your skid? Can you provide more detail about your situation?