As far as human is concerned Frieren was alive before reliably recorded history, and will go on to live so long for generations to come. To see her commenting "Elves are on a slow decline to extinction" I first feel sad, and then realize it'd be so slow it would be unimaginable to me.
Frieren will probably live at least another 1,000 years. Will Frieren still be alive in 10,000 years? Will she have overcome elves' lack of passion at least once and have an offspring during that time? If so, will her offspring goes on to live another 10,000 years? If elves will be going extinct, it'll still be a long, long time in the future.
EDIT:
Next week will be the episode with the same name as the series title: "Sousou no Frieren".