I've seen lots of photos of SFX Cupcake Pink in peoples hair but the results can vary quite a bit, it sometimes looks closer to Atomic Pink. How long would i have to leave it on to get somewhere near this colour: http://haircrazy.info/gallery/pink/698/
Still really bright but not too dark. I'll try a strand test but i'd like to hear your experiences with it, thanks :)

Half an hour or so should do it. It doesn't really develop as such. After about 10 minutes it's not really going to get any darker.
If you're worried about it going too dark why not mix in some toner or conditioner to lighten it. If it's too light reapply until you get the shade you like.

I use atomic pink and I always leave it on for about an hour. Like Jude says it's not gonna go too dark even if you leave it in for a while, that depends more on how dark the hair was before you put the pink in. But I always convince myself that the longer you leave it in the longer it lasts, I don't know if I'm right but at least it gets bright :)