Charity Majors:
Every job I’ve ever taken, I knew within the first week whether it was right for me or not. That might be overstating things a bit (memory can be like that). But I definitely had a strong visceral reaction to the company within days after starting, and the rest of my tenure played out more or less congruent with that reaction.
This has been true for me, too. I always try to get a sense of what a company will be like during the interview, but sometimes the impression isn’t accurate. Once, I took a job that seemed interesting. By the end of the first day, I knew it was a bad fit. I tried to give it some time, but my first impression was right.