Assuming that you are okay with someone moving on to a new opportunity eventually, how long do expect them to stay before you wouldn't consider them leaving to

Assuming that you are okay with someone moving on to a new opportunity eventually, how long do expect them to stay before you wouldn't consider them leaving to be damaging to their reputation? I still think you aren't fully taking into consideration the lack of symmetry of power in the employer-employee relationship and are expecting too much from employees while guaranteeing them so little.