It took me a few hours of work over a couple days to get about 600 of the stars up on my ceiling. I've created a few little galaxies and clusters of stars, as well as putting up my star sign and some other well known constellations.
It's been almost a month since I put them up and none have started peeling or anything. It took quite a bit of effort to pick off some badly placed stars as they adhere well, so they should stay up there for years. The ceiling is white and textured so in normal daylight the stickers are invisible unless you are actively looking for them.
I also have a blacklight which illuminates the stars brilliantly and leaves them glowing 10x brighter and longer than my normal lightbulb when switched off. In fact, I only need to flick the blacklight on for three seconds to leave the stars visible for upwards of half an hour (I tend to fall asleep before they completely fade).
My normal lightbulb leaves them visible in the pitch darkness but nowhere near as good as the blacklight.
To any parents considering decorating their childs ceiling, I would recommend getting a lamp and a blacklight bulb to charge the stars up before bed time as it makes such a huge difference in brightness and length of visiblity.