2 weeks.

Everyone feels more critical than they actually are. And then most people that read what I just said will think "Yeah, that sounds right, but that's not me. I am critical.".

They're wrong. They're not that important.

Sure, your current role may be extremely important.... but you can be replaced. Your knowledge can be dumped into documentation in less than 2 weeks. Even if you don't think it can (I didn't). Then the company can take their time training someone else to fill the hole you left, with your documentation to guide them.

And even if you gave 0 days notice, with 0 documentation? You'd burn a bridge.... but the company would be fine. An engineer having to reverse engineer all the crazy shit some ex-employee did over several years is not an unusual task. It's actually a very valuable skill to have, because that extends to legacy code. Imagine trying to deal with a 20 year old code base where all the original developers are either retired or dead. You can't ask questions, you have to reverse engineer it.

It's natural to think you need to give more than 2 weeks because you're important... but in reality 2 weeks is enough. That's why it's the norm.

I felt this exact same way after leaving a job I worked at for almost 4 years. I was a tech lead for 2 different teams, and the sole-developer for one R&D style project. Needless to say... I was a pretty critical player, and I had a lot of shit stored in my head.

When I left that company I thought the same way. I wanted to leave on good terms. So I gave 3 weeks notice. I was twiddling my thumbs doing nothing by the end of the 1st week. I absolutely didn't need to give that much notice, despite how "critical" I appeared to be.

Turnover is a very common thing in the business world. You aren't the first person to leave, and you won't be the last.