Stardate: 50973 The Doctor's life-extension experimental treatment sends Kes out of temporal sync. She begins traveling backward through time from the moment of her death. With each shift, she comes closer to a solution but she also grows months and years younger at a time.
Or she just "let it grow" instead of regularly trimming it. What's "2-3 years worth of hair" to a species that only lives 9 years? Human hair growth speeds would be irrelevant to a non-human.
Somewhere between "Darkling" (2 episodes back) and this episode, Kes apparently grows 2-3 years worth of hair? That made the time travel part even more confusing... I assumed she was starting at a more future point due to her much longer hair.