I consider the whole state of being a Force ghost akin to being a "real world" ghost -- ie. a spirit unnaturally trapped on the physical plain. While here, they may exist for a long, long time, but they aren't truly immortal and can eventually die/be destroyed.

I think that the only way a Force ghost can achieve true immortality is if he/she/it leaves the physical world completely behind (no more chatting with old apprentices) and completes the journey to the afterlife (becomes one with the Force, in other words).