After how long is a JW called inactive? Does anyone know for sure after how long of being inactive is a Jehovah's Witness announced officially as being inactive?
Inactive means the person has stopped sharing in our primary vocation, the preaching of the Good news of The Kingdom (Matthew 24:14)
This means they are not talking to non witnesses about their faith. There is no announcement made regarding this. 

I can’t find a shareable source  right now but if a baptized witness fails to report any witnessing activity for 6 consecutive months he is considered inactive.   

The subject of inactivity is addressed in the about us section of our website. 
Here we find that the number of Jehovah’s Witnesses reported in a given year comes from each country’s peak or highest month of persons reporting participation in the preaching activity. So if an individual is not reporting preaching activity during their country’s peak month they are not part of that years total number of active JW’s.

Whether one has stopped preaching for months or years, after being baptized, the only time an announcement is made regarding a person no longer being one of Jehovah’s Witnesses is if they are disfellowshipped for unrepentant serious sin or they disassociate themselves by requesting to no longer be identified as one of Jehovah’s Witnesses.

Being inactive does not mean the person is no longer one of Jehovah’s Witnesses. In fact special effort is made to reach out to those who have drifted away like this brochure entitled Return to Jehovah