The criteria that triggers this message has been outlined in the past. Regularly you can find information regarding a particular topic by just using the Forum Search function. I'm horrible at using the Search function, but I just searched for "This user recently woke up from a long period of inactivity." and it warranted this :
It shows up if a user has logged in with their last login time being at least 6 months ago.
I believe the message remains for 30-days after it has been triggered, but I will have to do a bit more reading/research before I can say definitively.