I know they linger if conditions are right. For example if the plane is flying through other cirrus clouds at the same level, the vapor trails will persist as long as the rest of the clouds do.
In any case, there are many clouds in the upper troposphere that are completely natural even if they look unnatural.
They do not sit in the stratosphere for hours and slowly spread out from horizon to horizon. That is Geoengineering for you.
Please post a picture, so I can analyze it.