Unfortunately it is not possible to make staff passwords expire after a certain number of days.
You could however place a prompt asking staff to change their password on the Dashboard Bulletin.
This can be done through Admin > Bulletin > Add New
This will let you create a message to put on the Dashboard for staff to see when they login. You can set a date on here as to how long it will show for also. Click Save once you've written your message.
When staff login they will then see this on their Bulletin section of the dashboard
For more information on the dashboard, click here.