As in right now. If you know you want to hire the candidate, make the offer. If this may be the right candidate, set the interview. Immediately. If you know you are going to need to fill a certain role, start the search. Right away.
Too many times a good hire doesn’t happen because someone drags their feet. Sometimes it’s a very short delay, but that’s all it takes. Desired candidates, those that fit important roles, are in high demand. If you don’t move then someone else will.
