Neptune's years are 164.8 Earth years. Its days are 16 hours and 6.7 minutes long. Although Neptune's days are shorter than our days, its years are still longer than ours.
Most colorants will last in your pond approximately 30 days. You may have to add more pond dye before 30 days if you have execissive rainfall, run-off, a heavy out flow, or evaporation. If your colorant is fading you will visually be able to tell and should add more dye to the water.
Re: How to make Henna/Indigo color last longer Is your henna from Mehandi/ Tap Dancing Lizard/ Ancient Sunrise, the company in Ohio? It is believable to me that a 6 hour application time for indigo might make it more permanent, so I'm glad you posted.