The way you charge your paint determines how bright it glows. Charging it the right way can give hours of glow while a wrong one may fade within an hour or a few minutes.
Now, direct sunlight and UV lights(like blacklight) is among the best chargers. They have the purest UV that gets easily absorbed by the pigments.
Remember, the source isn't a problem; its in the power of the source. If the power is less, then it will take a lot more time to charge.
You must keep your paint in the light and let them charge properly before you use them.