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.
After the dye completely disperses (usually around 4-6 hours), then the dye adjusts to what conditions the fish was previously accustomed to. On the other hand, if the dye is used for the first time, then an adjustment period would be needed for the fish to adjust to the change in the depth of color in the pond.