Depends how you count it surely?
After some creative revision Jean died in the shuttle crash before she became the Phoenix, her real body was put into stasis and the Phoenix ran around pretending to be her before she eventually died on the moon... that adds another 2 or three years to the vacuum between her death in ... holy shit. 1980 to 1986? Time used to seem longer. the ERA between the death of the Dark phoenix and X-Factor #1 used to seem immense... Although i was reading that all retroactively from quarter bins.
76 - 86, fuck the months, that's ten years.
Fake Magneto killed her in 2004, which makes it an 8 year stretch.