Yeah, 100% coolant will freeze rather quickly.
I'm assuming you're using good, high quality coolant, like Mopar 05066386AA or equivalent (Glysantin G05, Zerez G05, Zerex G48, BASF AG, or the Euro Peak stuff, all MB Spec 325.0)
A 50/50 mix will get you down to -34 degrees F.
A 60/40 mix...