As someone who has dealt with multiple people leaving (two fired, two quit with no warning, and two with warning), I honestly don't see much value after the first couple days. So honestly, a 1-week period would be plenty, if only to give HR a chance to properly close everything out while you're still easily reachable.

Even a month sounds excruciatingly long. We have a 2-week expectation here in the US, and it's more than sufficient to get someone off-boarded, though insufficient to find a replacement. And that's fine, we just adjust to whatever the new headcount is (usually by cutting out a bit of work after reassigning more important work).

That said, I would appreciate some form of mandatory severance. We don't have any, and it sucks when the market is poor.