We always let the player decide for himself. Lore-wise we simply said: "**** has heard the call of the undying lands. His time in Middle-Earth comes to a close."
The player would incorperate this into his character. He would make preparations, say goodbye to his kin, throw a party. Only once did a player leave and he was done with his character and wanted to play another one.
It could be 100 or 200 years, it all depends on the wish of the player.