How long for concrete to cure....?

So this is more of an argument. Pouring 5 inch thick slab for garage floor...current temperatures just a few degrees above freezing and the garage, while covered won't be much warmer. I say, it should cure longer than 48hrs in order to really ensure it will last long. My friend says that as long as it isn't freezing, it doesn't matter if it is 40F or 60F....it will cure at the same rate. Who is right ? thanks!