I also have it, can't give an accurate answer because I rotate the case I have on my phone every two weeks or so to keep things fresh.
What I can tell you is this:
If you are worried about it turning grimy black, do NOT get midnight blue. If you want a blue that'll hold its own for a a long time, get Sapphire, you won't regret it, Sapphire is also a limited edition colour that probably won't be around for much longer either so yeah