The best time to evolve in X/Y (and probably only X/Y) would be level 49.
This is because at that level it learns Play Rough, which is a good fairy attack, and is not worth missing out on.
However, if you're Skitty has Normalize, it won't matter, and you're good evolving it off straight away, you can give it Foresight and Return for power and Fake Out for residual damage, while having Charm to drop their attack.
However, there's this dreadful fact that Delcatty is nowhere near powerful with 65 base attack, most people won't use it at all, but meh, always use your favourites, nobody likes regretting it later.
Hope I helped!