Charge in full sun if you can, for most of the day is ideal, but I find that even an hour of indirect light is enough to charge these glow scales significantly. Without the sun, a flashlight will function fine, try for 20+ minutes.
The scales glow is very strong and hold their charge better than any glow-in-the-dark material I have found!
For a brighter, but short lived glow, try also charging in a blacklight (UV light) or lasers!