Phase Door, in core, lasts until used. You can pepper your halls with this thing, casting it during downtime, and then if/when combat breaks out, you can basically walk through all the walls you want. It also serves as a wonderful hiding place, as unless you are ejected via a successful dispel, you're on the ethereal plane inside a solid material object's location, which makes it hard to be attacked. Think of it as a location-linked contingent Ethereal Jaunt with multiple uses that you can bring your friends through.
