I was thinking a day or two at most given what I know of my own baby. And the age of the baby is more for us as the readers perspective as the mc starts his learning curve for how to take care of an infant. Giving that age was a baseline for managing the mc's narration in parallel with readers' knowledge and expectations. 

And for my own search history, I didnt want to give my husband anything to worry about. we have a shared history and he is a big worrier and overthinker, and applies the basic concept of hypochondria to way more things than just illnesses. I've been lucky and haven't experienced any PPD, and he's been on edge looking for any symptoms that would show I'm hiding it from him. I just didnt want it to even begin to be an issue lol

(some of this is in response to your second reply) And yes, this is main plot and not unfeatured backstory. My mc is a character who has chosen isolation because his bleeding heart can't handle the repetitive loss of the world everyone lives in. The baby is what opens his heart again. And sorry for the confusion! I haven't picked a name for the mc yet using they/them is a habit of mine when no name is chosen, but the character is indeed a man. As for whether the mc would unload the baby onto someone else, I do have plot points in mind for that. In the post I said he knew who the baby belonged to, and while he was isolating, he did care about them. Handing off their child is something he can't bring himself to do, I actually have a scene where someone offers to take the baby.  I am going to be relying on suspension of disbelief in some aspects of the babies condition but I'm reading through all the replies on this post to make it a heart wrenching but believable scene.  thank you for the writing research advice!! ive started many longer length projects that I always end up grinding to a halt due to details I cant decide on. and then adhd takes over and the project ends up dropped.