Fresh news: to remedy the issue with "respotters", I just made some changes in the artillery code. In the next version of FH2, if a player puts another spot, every gunner who has the previous spot of that player as active, will be switched to the new spot, without intermediate switching to the first spot in the queue and other annoyances. Still, it won't save the gunners from readjusting their guns if the new spot is, say, hundreds meters away from the previous one.
As for when spots disappear: normally spots live for 2 minutes, but if their spotters die or pick up a kit, spots disappear in about 30-60 secs.