I suppose every company or team is different. But almost everyone on my team told me they are moving on about the same time they started looking.

This is beneficial both ways:

* A smooth transition can be made. They get to wrap up their last project at the company and have enough time to do a proper knowledge transfer

* We get a head start on hiring a replacement. Typically the person leaving gets to help out during the interview process as well.

* I get to help the person leaving on picking out the right next opportunity and more often than not, help the person negotiate his next job offer.

There is nothing wrong in looking after other opportunities, just like as an employer you would constantly look after new candidates, welcome to 2013! Manager will assume that you have something else lineup up already anyway.