In my experience, it takes somewhere between eighteen months and two years to see the bones of a company. What it’s really like, what drives it.
Doubts will likely surface within a year if something doesn’t fit with you. It takes a few cycles to see beneath the surface.
By this point, you can almost always tell whether it is capable of change, where it’s priorities are.
The further up the chain you trust, the longer you will usually stay.