I mean the answer is that you just feel it out. 1 month is soon enough to wait, 2 months is gray area, 3 months is probably fine. And if it's a huge vest, then you give more leeway.

You're not appealing to some objective rule here. You're thinking about what would look like shitty behavior to the rest of your employees.

Firing people gracefully is a really, really important part your relationship to your employees. It's about whether people feel like they're being respected as professionals or abused like they work in a sweatshop.