I have resigned for two different jobs where I was the highest tech leader, reporting to the CEO (basically CTO role without the title). Both times I gave 1 month of notice. Both times all the "transition" work was done during the first 2 weeks, and afterwards, I basically sat at meeting listening without say (I pushed for my "replacement" to be the one making the decisions as if I wasn't there) and even the CEO asked me to stay at home in one of the two jobs.
My thought is that if that worked for me for 2 weeks, it should also be more than enough for an IC.
I don't know. Not to offend you but in general I consider leadership roles to be more easily replaceable than IC ones. Fundamentally, it's the ICs that usually have all the intricate knowledge of the details. That is something I'd argue cannot be handed over in just two weeks, in particular to just anyone.
On the other hand, the leadership folks I interact with always make only super high level decisions. Rarely does it get intricate. It's more important to know how to quickly assess the big picture and how to communicate. All lot of what a leader does and makes them uniquely leaders is not something that's based on acquiring company specific and product and infra specific knowledge accumulated across several years.
I rarely care if my manager leaves beyond the fact that I have to build trust with someone new. But ultimately they aren't super in the weeds on anything mission critical.
An engineer that can jump into an incident and immediately identify the problem on a code path because they have worked on it or around it intimately at some point does make a difference, but also just having a detailed mental model of how the system pieces interact is super crucial. Bus factor is a thing, even though it rarely is catastrophic. Lead time can help here.
Of course leaders leaving also is a challenge, but it's because of their unique charm, ability to grasp issues quickly and make sane decisions. But none of this can be transferred to a new guy, in 2 or 4 weeks or 8.
My thought is that if that worked for me for 2 weeks, it should also be more than enough for an IC.