You’re coaching an employee about their performance. How much time do you give them to improve? It depends on whether it’s a Skillset issue or a Behavioral issue.
Skillset issues can be resolved in a reasonable timeframe based on the deficiency and what a reasonable person would need to improve. This could be: 2 weeks of job shadowing, re-watching a training course, taking an outside class, or hands-on training in-house.
Behavior issues have a very different timeline.
We take the position that humans can control their own behavior. Sometimes even “mature” adults seem to forget this. The truth is we do have control over the words we use, our tone, whether we speak or not, whether or not we roll our eyes, how combative or accommodating we are, etc.
There are certainly different personalities with differing levels of introversion/extroversion, dominance/conforming, and so on. Nonetheless, one’s personality does not cause them to be a jerk to their coworkers.
If you are coaching an employee on their behavior, it is reasonable to expect to see improvement starting tomorrow. An adult doesn’t need 3 weeks to learn how to not roll their eyes at a coworker, nor do they need 5 weeks to figure out how to get up earlier and be on time. In your meeting with the employee, make it clear that you believe behavior can change right away, and that you expect to see positive changes starting immediately. It is reasonable to give the employee a very short timeline (such as a few days or a week) to improve.