I think unless you're going for a radioactive or blacklight-using solution, any glow-in-the-dark paint is going to need to charge in the light. Sunlight is apparently most effective, but indoor lighting can do the trick, too, as long as you're not doing something like only leaving the lights on for 3 minutes and hoping for much glow effect afterward.
I just recently got introduced to
glowinc.com which sells a lot of glow-in-the-dark stuff. I haven't tried their paint yet, but the powder and rocks I got with a similar "brightness rating" I'm pretty happy with. The glow definitely goes down after a few minutes, but once my eyes are adjusted to the dark they're still visible for hours.
On the forums there people talk about using UV lights to better "charge" their glow-in-the-dark materials, but in my brief recent search I didn't see much that seemed extremely cheap and hassle-free. I don't want to go wall-mounting anything.