Fourteen years to a sign. Slower than a generation.
Neptune takes about fourteen years to cross a sign, so its placement identifies a birth era and very little else about a person.
Its discovery is one of the better stories in astronomy: Le Verrier computed where an unseen planet must be to explain Uranus's wobble, sent the coordinates to Berlin, and it was found the same night within a degree of the prediction.
Modern astrology gives it Pisces; the traditional scheme gives Pisces to Jupiter, and both readings are current.
What it is
Found in 1846 by prediction rather than survey — its position was calculated from irregularities in Uranus's orbit before anyone looked.