Ahhhh I think you have just answered my question thank you.  It was only about 1-2 days.

Judging by your question did I not wait long enough for any gas/vapours coming off the paint to dissipate before applying the decal?  It would explain too why it didn't bubble immediately but only showed up on about the 3 day, I'm guessing by then enough had accumulated to force the decal to rise?