The amount of glow depends upon one's perception. The powder will glow for over 12 hours, but how well you see the glow depends upon a number of factors. If there is ambient light, your eyes will have a tendency to adjust for that ambient light. If your eyes are not well adjusted to the dark, you may have trouble seeing the glow. If you get up in the middle of the night and walk into a dark room with the powder, you will see it very well. If you come from outside on a sunny day and walk into the same room, the glow will be very bright but you will not see the glow until your eyes adjust to the darkness. In dark room, by emitting-light from fully-charged LightLead's PL pigment in a bottle(diameter 35mm, height 50mm), the 12# Times New Roman characters are readable for one hour at a distance of 10cm. And it has been scientifically tested to emit light for up to 21 days in laboratory conditions.