Whenever someone leaves a position for a new position, there are three parties who are effected. In the marketing agency world, turnover is a common issue so this is especially important. Here’s how each party should, conceptually, handle a departure:
If you are the employee, it is best to give as much notice as you can comfortably give to your current employer without putting yourself in a vulnerable position.
If you are the hiring company, you want to get the new employee onboard as quickly as possible, but don’t put the person you are hiring in an awkward and damaging position.
If you are the company losing the employee, get a reasonable transition time, find some way to manage without the person, and don’t try to keep them once they’ve shown a desire to leave.
I believe the Harvard case for an exiting employee goes something like this:
If you decide to leave due to underlying issues and your manager counter offers, you should reject this offer. The underlying issues will not go away within a reasonable time frame and you will be left at an employer you do not want to be at and you will have burnt your bridge with the potential employer who made an offer.