With the Portable Licence Utility there is an "emergency license". It is not to be misused and cannot be used every day. How long it takes before you can use it again is not documented or revealed by Autodesk.
When can this be useful? Say you have exported a license to your home PC and in last minute find that you need to work overtime. There is no need to panic, having to go home and back to work again. In a case like this you can use the emergency license.