I don't rely much on first impressions. I used to, but I got proved wrong in too many instances.
However, I don't think it takes a person's true colors too long before they're seen. Before I retired, I was 21 years with the same company, and saw a lot of employees come and go in that time. I eventually went more with the "wait and see" option, since it doesn't take too long to separate the people who are there to work and do a good job, from the ones that are there only for the sake of a paycheck.