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.
I would imagine most semi-and demi-permanent dyes are about the same. If you are using a vegetable dye (such as manic panic, special effects, or henna) you may leave on as long as you want, from 30 minutes to overnight.