What is an acceptable time limit to Move on depends on how the split eventuated and judgemental people have no right to say poor children when they could of witnessed a lot of dmamging stuff than the new relationship
I keep seeing this idea that because a bad marriage can be damaging, which is very true, that this somehow justifies and erases the concerns people have about quick parent replacements/additions. Personally, in my experience, I think the more traumatic things a child has experienced, the more important it to for any dating or similar is in the child's periphery to help ensure a child's stability is prioritised as many have described.
Back when and where I was a child there was a lot of judgement in staying single with children - I was 'that poor child' with a lot of trauma having dealt with being around drug use and subjected to a lot of violence. A lot of people around my father thought what my siblings and I needed was a mother figure and that it would be best for all of us to 'move on'. My father was living it up and dating well out of my knowledge at first (I have court records involving some partying related arrests that I didn't know about until I was an adult to prove it), but is a man easily led by others' opinions and so followed their advice of bringing his girlfriend into the family. And then the next one. He thankfully stopped after that for my younger sister's and his own sake (to this day, he's a partying single man - that's his life), but I was gone by then. I don't know what that was meant to help, but it caused a lot of damage, some of which still isn't repaired. We didn't have two loving homes - my mother wanted nothing to do with us most of the time, me especcially - we had one shaky one. No amount of pictures on the wall was going to make my mother an "exparent" or erased what happened, it just led me to feeling like I couldn't talk about it for fear of being erased entirely as well.
That advice didn't work well for many which has swung advice and judgement the other way -- yeah it's not nice and sometimes it goes too far or seems smug, but I prefer the current more cautious advice that's going around now on bringing in partners especially as a parenting figure. When things are rough, bringing in an additional risk factor often makes it worse and someone new acting as a parent is an additional risk factor for the child.
How soon is too soon depends on a lot of factors -- sometimes the safest option is moving in straight away with another adult, but I've only seen that done well when there is a lot of effort in the 'new partner' not taking on a parenting role for a while, being almost like a lodger to the child and letting the child make the first moves towards getting closer and the other parent - even when not wanting anything to do with the child or not available, isn't treated like an "exparent". My mother was ruthlessly violent to the point of trying to kill me, she still wasn't and never will be my ex-mother and no amount of pictures will ever change that reality. I needed support moving forward, not moving on - and I needed stability, not a picture perfect 2 parent household, to do that.